Portable Document Format (PDF) is a
file format used to represent
documents in a manner independent of
application software,
hardware, and
operating systems.[1]
Each PDF file encapsulates a complete description of a fixed-layout flat
document, including the text, fonts, graphics, and other information
needed to display it. In 1991,
Adobe Systems co-founder
John Warnock outlined a system called "Camelot"[2]
that evolved into PDF.
While Adobe Systems made the PDF specification available free of
charge in 1993, PDF remained a
proprietary format, controlled by Adobe, until it was officially
released as an
open standard on July 1, 2008, and published by the
International Organization for Standardization as ISO 32000-1:2008.[3][4]
In 2008, Adobe published a Public Patent License to ISO 32000-1 granting
royalty-free rights for all patents owned by Adobe that are
necessary to make, use, sell and distribute PDF compliant
implementations.[5]
History
PDF was developed in the early 1990s as a way to share documents,
including text formatting and inline images, among computer users of
disparate platforms who may not have access to mutually-compatible
application software.[6]
It was among a number of competing formats such as
DjVu (still
developing),
Envoy,
Common Ground Digital Paper,
Farallon Replica and even
Adobe's
own
PostScript format (.ps). In those early years before the rise of the
World Wide Web and
HTML
documents, PDF was popular mainly in
desktop publishing
workflows.
PDF's adoption in the early days of the format's history was slow.[7]
Adobe Acrobat, Adobe's suite for reading and creating PDF files, was
not freely available; early versions of PDF had no support for external
hyperlinks, reducing its usefulness on the Internet; the larger size of
a PDF document compared to plain text required longer download times
over the slower
modems
common at the time; and rendering PDF files was slow on the less
powerful machines of the day.
From version 2.0 onwards Adobe distributed its Acrobat Reader (now
Adobe Reader) program free of charge,[8]
and continued supporting the original PDF, which eventually became the
de facto standard for printable documents on the web[citation
needed] (a standard
web document).
In 2008 Adobe Systems' PDF Reference 1.7 became ISO 32000:1:2008.
Thereafter, further development of PDF (including PDF 2.0) is conducted
by ISO's TC 171 SC 2 WG 8 with the participation of Adobe Systems' and
other subject matter experts.
Adobe's
PDF specifications
Adobe changed the PDF specification a few times and continues to
develop new specifications with new versions of Adobe Acrobat. There
have been nine versions of PDF with corresponding Acrobat releases:[9]
- 1993 – PDF 1.0 / Acrobat 1.0
- 1994 – PDF 1.1 / Acrobat 2.0
- 1996 – PDF 1.2 / Acrobat 3.0
- 2000 – PDF 1.3 / Acrobat 4.0
- 2001 – PDF 1.4 / Acrobat 5.0
- 2003 – PDF 1.5 / Acrobat 6.0
- 2005 – PDF 1.6 / Acrobat 7.0
- 2006 – PDF 1.7 / Acrobat 8.0
- 2006 – PDF 1.7 / Acrobat 8.2
- 2008 – PDF 1.7, Adobe Extension Level 3 / Acrobat 9.0
- 2009 – PDF 1.7, Adobe Extension Level 5 / Acrobat 9.1
The ISO standard
ISO 32000-1:2008 and Adobe PDF 1.7 are technically consistent.[10]
Adobe declared that it is not producing a PDF 1.8 Reference. Future
versions of the PDF Specification will be produced by ISO technical
committees. However, Adobe published documents specifying what extended
features for PDF, beyond ISO 32000-1 (PDF 1.7), are supported in its
newly released products. This makes use of the extensibility features of
PDF as documented in ISO 32000-1 in Annex E.
The specifications for PDF are backward inclusive. The PDF 1.7
specification includes all of the functionality previously documented in
the Adobe PDF Specifications for versions 1.0 through 1.6. Where Adobe
removed certain features of PDF from their standard, they are not
contained in ISO 32000-1[3]
either.
PDF documents conforming to ISO 32000-1 carry the PDF version number
1.7. Documents containing Adobe extended features still carry the PDF
base version number 1.7 but also contain an indication of which
extension was followed during document creation.[11]
Adobe's versions
Version |
Edition[3] |
Year of publication |
New features |
Acrobat Reader version support |
1.0 |
First |
1993 |
|
Carousel |
1.1 |
First, revised |
1996 |
Passwords, encryption (MD5, RC4 40bit), device-independent
color, threads and links[12] |
2.0 |
1.2 |
First, revised |
1996 |
Interactive page elements (radio buttons, checkboxes &c);
interactive, fill-in forms (AcroForm); Forms Data Format (FDF)
for interactive form data that can be imported, exported,
transmitted and received from the Web; mouse events; external
movie reproduction; external or embedded sound reproduction;
zlib/deflate
compression of text or binary data; Unicode; advanced color
features and image proxying[12] |
3.0 |
1.3 |
Second |
2000 |
Digital signatures;
ICC and DeviceN color spaces; JavaScript actions; embedded
file streams of any type (e.g. used for attachments); new
annotation types; new features of the Adobe PostScript Language
Level 3 imaging model; masked images; alternate representations
for images; smooth shading; enhanced page numbering; Web capture
— a facility for capturing information from World Wide Web and
converting it to PDF; representation of logical structure
independently of graphical structure; additional support for
CIDFonts; data structures for mapping strings and numbers to PDF
objects; information for prepress production workflows support;
new functions for several function object types that represent
parameterized classes of functions[13][14] |
4.0 |
1.4 |
Third |
2001 |
JBIG2; transparency; RC4 encryption key lengths greater than
40 bits (40–128 bits); enhancements to interactive forms and
Forms Data Format (FDF), XML form submissions, embedded FDF
files, Unicode specification of field export values, remote
collaboration and digital signatures in FDF files; accessibility
to disabled users; metadata streams using XML — Extensible
Metadata Platform (XMP); tagged PDF; inclusion of printer’s
marks; display and preview of production-related page
boundaries; new predefined CMaps; alternate presentations;
importing content from one PDF document into another;
EmbeddedFiles entry in the PDF document’s name dictionary — a
standard location for the embedded data;[14][15]
OCR text layer[citation
needed] |
5.0 |
1.5 |
Fourth |
2003 |
JPEG 2000; enhanced support for embedding and playback of
multimedia; object streams; cross reference streams; XML Forms
Data Format (XFDF) for interactive form submission (replaced the
XML format in PDF 1.4); support for forms, rich text elements
and attributes based on Adobe’s XML Forms Architecture (XFA)
2.02; public-key security handlers using
PKCS#7 (introduced in PDF 1.3 but not documented in the
Reference until 1.5), public-key encryption, permissions — usage
rights (UR) signatures (does not require document encryption),
PKCS#7 with SHA-1, RSA up to 4096-bits; security handler can use
its own encryption and decryption algorithms; document sections
selectively viewed or hidden by authors or readers — for items
such as
CAD drawings,
layered artwork, maps, and multi-language documents;
Alternate Presentations — the only type is slideshow — invoked
by means of JavaScript actions (Adobe Reader supports only
SVG 1.0);[14][16][17]
support for MS
Windows 98 dropped. |
6.0 |
1.6 |
Fifth |
2004 |
3D artwork, e.g. support for
Universal 3D file format;
OpenType font embedding; support for XFA 2.2 rich text
elements and attributes;
AES encryption; PKCS#7 with SHA256, DSA up to 4096-bits;
NChannel color spaces; additional support for embedded file
attachments, including cross-document linking to and from
embedded files; enhancements and clarifications to digital
signatures related to usage rights and modification detection
and prevention signatures[14] |
7.0 |
1.7
(ISO 32000-1:2008
[3][11]) |
Sixth (ISO first) |
2006 |
Increased presentation of 3D artwork; XFA 2.4 rich text
elements and attributes; multiple file attachments (portable
collections); document requirements for a PDF consumer
application; new string types: PDFDocEncoded string, ASCII
string, byte string; PKCS#7 with SHA384, SHA512 and RIPEMD160 |
8 |
1.7 Extension Level 3 |
|
2008 |
256-bit
AES encryption; incorporation of XFA Datasets into a file
conforming PDF/A-2; improved attachment of Flash applications,
video (including Flash video with H.264), audio, and other
multimedia, two-way scripting bridge between Flash and
conforming applications; XFA 2.5 and 2.6 rich text conventions |
9 |
1.7 Extension Level 5 |
|
2009 |
XFA 3.0 |
9.1 |
1.7 Extension Level 8[18] |
|
2011 |
Specification not published as of May 2011.[11]
AES-256 different password handling than in Extension Level 3.[19][20] |
X (10) |
Standardization
Specialized subsets of PDF
The following specialized subsets of PDF specification has been
standardized as ISO standards (or are in standardization process):[3][21][22][23]
- PDF/X
(since 2001 - series of ISO 15929 and ISO 15930 standards) - a.k.a.
"PDF for Exchange" - for the Graphic technology - Prepress
digital data exchange - (working in ISO Technical committee
130), based on PDF 1.3, PDF 1.4 and later also PDF 1.6
- PDF/A
(since 2005 - series of ISO 19005 standards) - a.k.a. "PDF for
Archive" - Document management - Electronic document file format
for long-term preservation (working in ISO Technical committee
171), based on PDF 1.4 and later also ISO 32000-1 - PDF 1.7
- PDF/E
(since 2008 - ISO 24517) - a.k.a. "PDF for Engineering" -
Document management - Engineering document format using PDF
(working in ISO Technical committee 171), based on PDF 1.6
-
PDF/VT (since 2010 - ISO 16612-2) - a.k.a. "PDF for exchange of
variable data and transactional (VT) printing" - Graphic
technology - Variable data exchange (working in ISO Technical
committee 130), based on PDF 1.6 as restricted by PDF/X-4 and
PDF/X-5[24]
-
PDF/UA (since 2012 - ISO 14289-1) - a.k.a. "PDF for Universal
Access" - Document management applications - Electronic document
file format enhancement for accessibility (working in ISO
Technical committee 171), based on ISO 32000-1 - PDF 1.7
There is also the PDF/H, a.k.a. PDF Healthcare, a best
practices guide (BPG), supplemented by an Implementation Guide (IG),
published in 2008. PDF Healthcare is not a standard or proposed
standard, but only a guide for use with existing standards and other
technologies. It is supported by the standards development organizations
ASTM and
AIIM. PDF/H BPG is based on PDF 1.6.[25][26][27]
Full Function PDF
PDF 1.7
The final revised documentation for PDF 1.7 was approved by ISO
Technical Committee 171 in January 2008 and published as ISO
32000-1:2008 on July 1, 2008 and titled Document management—Portable
document format—Part 1: PDF 1.7.
ISO 32000-1:2008 is the first ISO standard for full function PDF. The
previous ISO PDF standards (PDF/A, PDF/X, etc.) are intended for more
specialized uses. ISO 32000-1 includes all of the functionality
previously documented in the Adobe PDF Specifications for versions 1.0
through 1.6. Adobe removed certain features of PDF from previous
versions; these features are not contained in PDF 1.7 either.[3]
The ISO 32000-1 document was prepared by Adobe Systems Incorporated
based upon PDF Reference, sixth edition, Adobe Portable Document
Format version 1.7, November 2006. It was reviewed, edited and
adopted under a special fast-track procedure, by ISO Technical
Committee 171 (ISO/TC 171), Document management application,
Subcommittee SC 2, Application issues, in parallel with its approval
by the ISO member bodies.
According to the ISO PDF standard abstract:[28]
ISO 32000-1:2008 specifies a digital form for representing
electronic documents to enable users to exchange and view electronic
documents independent of the environment they were created in or the
environment they are viewed or printed in. It is intended for the
developer of software that creates PDF files (conforming writers),
software that reads existing PDF files and interprets their contents
for display and interaction (conforming readers) and PDF products
that read and/or write PDF files for a variety of other purposes
(conforming products).
PDF 2.0
A new version of PDF standard is under development under the name
ISO/CD 32000-2 - Document management—Portable document format—Part 2:
PDF 2.0 (as of August 2013).[29][30]
PDF 2.0 was accepted by ISO as a new proposal in 2009 (ISO/NP 32000-2).
The TC 171 SC 2 WG 8 Committee working on ISO 32000-2 (PDF 2.0) is
continuing to actively develop the document; processing hundreds of
technical and editorial comments and operating eight ad hoc
committees comprising numerous interested parties, including Adobe
Systems. To provide more time to develop the document the original ISO
project was cancelled in 2012 (in the "enquiry stage" as a "Draft
International Standard") and a New Project item was started.[30][29][citation
needed]
Adobe has submitted the Adobe Extension Level 5 and Adobe
Extension Level 3 specifications to ISO for inclusion into the ISO
32000-2 specification, but only some of their features have been
accepted.
PDF 2.0 will reference Adobe's XML Forms Architecture 3.1. In 2011
the ISO Committee urged Adobe Systems to submit the XFA Specification,
XML Forms Architecture (XFA), to ISO for standardization and requested
Adobe Systems to stabilize the XFA specification. The ISO Committee
expressed its concerns about the stability of the XFA specification.[31]
ISO TC 171 SC 2
WG 8
Formed in 2008 to curate the PDF Reference as an ISO Standard,
Working Group 8 typically meets twice a year, with members from ten or
more countries attending in each instance. Meetings of the ISO Committee
for ISO 32000 are open to accredited Subject Matter Experts. Interested
parties should contact their respective ISO Member Body for information
about joining ISO 32000.
Current Project Leadership: Cherie Ekholm,
Microsoft & Duff Johnson,
Independent Consultant, Project Co-Leaders
Past Project Leadership: 2008-2011: James King, PhD,
Adobe Systems
Secretary: Betsy Fanning,
AIIM
Technical
foundations
Anyone may create applications that can read and write PDF files
without having to pay royalties to
Adobe Systems; Adobe holds patents to PDF, but licenses them for
royalty-free use in developing software complying with its PDF
specification.[32]
The PDF combines three technologies:
- A subset of the
PostScript page description programming language, for generating
the layout and graphics.
- A font-embedding/replacement system to allow fonts to travel
with the documents.
- A structured storage system to bundle these elements and any
associated content into a single file, with
data compression where appropriate.
PostScript
PostScript is a
page description language run in an
interpreter to generate an image, a process requiring many
resources. It can handle not just graphics, but standard features of
programming languages such as if
and loop
commands. PDF is largely based on PostScript but simplified to remove
flow control features like these, while graphics commands such as
lineto
remain.
Often, the PostScript-like PDF code is generated from a source
PostScript file. The graphics commands that are output by the PostScript
code are collected and
tokenized; any files, graphics, or fonts to which the document
refers also are collected; then, everything is compressed to a single
file. Therefore, the entire PostScript world (fonts, layout,
measurements) remains intact.
As a document format, PDF has several advantages over PostScript:
- PDF contains tokenized and interpreted results of the PostScript
source code, for direct correspondence between changes to items in
the PDF page description and changes to the resulting page
appearance.
- PDF (from version 1.4) supports true
graphic transparency; PostScript does not.
- PostScript is an
interpreted programming language with an implicit global state,
so instructions accompanying the description of one page can affect
the appearance of any following page. Therefore, all preceding pages
in a PostScript document must be processed to determine the correct
appearance of a given page, whereas each page in a PDF document is
unaffected by the others. As a result, PDF viewers allow the user to
quickly jump to the final pages of a long document, whereas a
Postscript viewer needs to process all pages sequentially before
being able to display the destination page (unless the optional
PostScript
Document Structuring Conventions have been carefully complied
with).
Technical overview
File structure
A PDF file consists primarily of objects, of which there are
eight types:[33]
-
Boolean values, representing true or false
- Numbers
-
Strings
- Names
-
Arrays, ordered collections of objects
-
Dictionaries, collections of objects indexed by Names
-
Streams, usually containing large amounts of data
- The
null object
Objects may be either direct (embedded in another object) or
indirect. Indirect objects are numbered with an object number
and a generation number. An index table called the xref table
gives the byte offset of each indirect object from the start of the
file.[34]
This design allows for efficient
random access to the objects in the file, and also allows for small
changes to be made without rewriting the entire file (incremental
update). Beginning with PDF version 1.5, indirect objects may also
be located in special streams known as object streams. This
technique reduces the size of files that have large numbers of small
indirect objects and is especially useful for Tagged PDF.
There are two layouts to the PDF files—non-linear (not "optimized")
and linear ("optimized"). Non-linear PDF files consume less disk space
than their linear counterparts, though they are slower to access because
portions of the data required to assemble pages of the document are
scattered throughout the PDF file. Linear PDF files (also called
"optimized" or "web optimized" PDF files) are constructed in a manner
that enables them to be read in a Web browser plugin without waiting for
the entire file to download, since they are written to disk in a linear
(as in page order) fashion.[11]
PDF files may be optimized using
Adobe Acrobat software or
QPDF.
Imaging model
The basic design of how
graphics are represented in PDF is very similar to that of
PostScript, except for the use of
transparency, which was added in PDF 1.4.
PDF graphics use a
device independent
Cartesian coordinate system to describe the surface of a page. A PDF
page description can use a
matrix to
scale,
rotate, or
skew graphical elements. A key concept in PDF is that of the
graphics state, which is a collection of graphical parameters that
may be changed, saved, and restored by a page description. PDF
has (as of version 1.6) 24 graphics state properties, of which some of
the most important are:
Vector graphics
Vector graphics in PDF, as in PostScript, are constructed with
paths. Paths are usually composed of lines and cubic
Bézier curves, but can also be constructed from the outlines of
text. Unlike PostScript, PDF does not allow a single path to mix text
outlines with lines and curves. Paths can be stroked, filled, or used
for
clipping. Strokes and fills can use any color set in the graphics
state, including patterns.
PDF supports several types of patterns. The simplest is the tiling
pattern in which a piece of artwork is specified to be drawn
repeatedly. This may be a colored tiling pattern, with the colors
specified in the pattern object, or an uncolored tiling pattern,
which defers color specification to the time the pattern is drawn.
Beginning with PDF 1.3 there is also a shading pattern, which
draws continuously varying colors. There are seven types of shading
pattern of which the simplest are the axial shade (Type 2) and
radial shade (Type 3).
Raster images
Raster images in PDF (called Image XObjects) are represented
by dictionaries with an associated stream. The dictionary describes
properties of the image, and the stream contains the image data. (Less
commonly, a raster image may be embedded directly in a page description
as an inline image.) Images are typically filtered for
compression purposes. Image filters supported in PDF include the general
purpose filters
- ASCII85Decode a filter used to put the stream into 7-bit
ASCII
- ASCIIHexDecode similar to ASCII85Decode but less compact
- FlateDecode a commonly used filter based on the
zlib/deflate
algorithm (a.k.a.
gzip,
but not
zip) defined in
RFC 1950 and
RFC 1951; introduced in PDF 1.2; it can use one of two groups of
predictor functions for more compact zlib/deflate compression:
Predictor 2 from the
TIFF 6.0 specification and predictors (filters) from the
PNG specification (RFC
2083)
- LZWDecode a filter based on
LZW Compression; it can use one of two groups of predictor
functions for more compact LZW compression: Predictor 2 from
the TIFF 6.0 specification and predictors (filters) from the PNG
specification
- RunLengthDecode a simple compression method for streams
with repetitive data using the
Run-length encoding algorithm and the image-specific filters
- DCTDecode a
lossy filter based on the
JPEG
standard
- CCITTFaxDecode a
lossless
bi-level (black/white) filter based on the Group 3 or
Group 4
CCITT (ITU-T)
fax
compression standard defined in ITU-T
T.4 and T.6
- JBIG2Decode a lossy or lossless bi-level (black/white)
filter based on the
JBIG2
standard, introduced in PDF 1.4
- JPXDecode a lossy or lossless filter based on the
JPEG 2000 standard, introduced in PDF 1.5
Normally all image content in a PDF is embedded in the file. But PDF
allows image data to be stored in external files by the use of
external streams or Alternate Images. Standardized subsets of
PDF, including
PDF/A and
PDF/X,
prohibit these features.
Text
Text in PDF is represented by text elements in page content
streams. A text element specifies that characters should be drawn
at certain positions. The characters are specified using the encoding
of a selected font resource.
Fonts
A font object in PDF is a description of a digital
typeface. It may either describe the characteristics of a typeface,
or it may include an embedded font file. The latter case is
called an embedded font while the former is called an
unembedded font. The font files that may be embedded are based on
widely used standard digital font formats:
Type 1 (and its compressed variant CFF),
TrueType, and (beginning with PDF 1.6)
OpenType. Additionally PDF supports the Type 3 variant in
which the components of the font are described by PDF graphic operators.
Standard Type 1 Fonts (Standard 14 Fonts)
Fourteen typefaces—known as the standard 14 fonts—have a
special significance in PDF documents:
These fonts are sometimes called the base fourteen fonts.[35]
These fonts, or suitable substitute fonts with the same metrics, must
always be available in all PDF readers and so need not be embedded in a
PDF.[36]
PDF viewers must know about the metrics of these fonts. Other fonts may
be substituted if they are not embedded in a PDF.
Encodings
Within text strings, characters are shown using character codes
(integers) that map to glyphs in the current font using an encoding.
There are a number of predefined encodings, including WinAnsi,
MacRoman, and a large number of encodings for East Asian languages,
and a font can have its own built-in encoding. (Although the WinAnsi and
MacRoman encodings are derived from the historical properties of the
Windows and
Macintosh operating systems, fonts using these encodings work
equally well on any platform.) PDF can specify a predefined encoding to
use, the font's built-in encoding or provide a lookup table of
differences to a predefined or built-in encoding (not recommended with
TrueType fonts).[37]
The encoding mechanisms in PDF were designed for Type 1 fonts, and the
rules for applying them to TrueType fonts are complex.
For large fonts or fonts with non-standard glyphs, the special
encodings Identity-H (for horizontal writing) and Identity-V
(for vertical) are used. With such fonts it is necessary to provide a
ToUnicode table if semantic information about the characters is to
be preserved.
Transparency
The original imaging model of PDF was, like PostScript's, opaque:
each object drawn on the page completely replaced anything previously
marked in the same location. In PDF 1.4 the imaging model was extended
to allow transparency. When transparency is used, new objects interact
with previously marked objects to produce blending effects. The addition
of transparency to PDF was done by means of new extensions that were
designed to be ignored in products written to the PDF 1.3 and earlier
specifications. As a result, files that use a small amount of
transparency might view acceptably in older viewers, but files making
extensive use of transparency could be viewed incorrectly in an older
viewer without warning.
The transparency extensions are based on the key concepts of
transparency groups, blending modes, shape, and
alpha. The model is closely aligned with the features of
Adobe Illustrator version 9. The blend modes were based on those
used by
Adobe Photoshop at the time. When the PDF 1.4 specification was
published, the formulas for calculating blend modes were kept secret by
Adobe. They have since been published.[38]
The concept of a transparency group in PDF specification is
independent of existing notions of "group" or "layer" in applications
such as Adobe Illustrator. Those groupings reflect logical relationships
among objects that are meaningful when editing those objects, but they
are not part of the imaging model.
Interactive
elements
PDF files may contain interactive elements such as annotations and
form fields.
Interactive Forms is a mechanism to add forms to the PDF file
format.
PDF currently supports two different methods for integrating data and
PDF forms. Both formats today coexist in PDF specification:[39][40][41][42]
- AcroForms (also known as Acrobat forms),
introduced in the PDF 1.2 format specification and included in all
later PDF specifications.
-
Adobe XML Forms Architecture (XFA) forms, introduced in the
PDF 1.5 format specification. The XFA specification is not included
in the PDF specification, it is only referenced as an optional
feature. Adobe XFA Forms are not compatible with AcroForms.[43]
AcroForms
AcroForms were introduced in the PDF 1.2 format. AcroForms permit
using objects (e.g.
text
boxes,
Radio buttons, etc.) and some code (e.g.JavaScript).
Alongside the standard PDF action types, interactive forms
(AcroForms) support submitting, resetting, and importing data. The
"submit" action transmits the names and values of selected interactive
form fields to a specified uniform resource locator (URL). Interactive
form field names and values may be submitted in any of the following
formats, (depending on the settings of the action’s ExportFormat,
SubmitPDF, and XFDF flags):[39]
- HTML Form format (HTML 4.01 Specification since PDF 1.5; HTML
2.0 since 1.2)
- Forms Data Format (FDF)
- XML Forms Data Format (XFDF) (external XML Forms Data Format
Specification, Version 2.0; supported since PDF 1.5; it replaced the
"XML" form submission format defined in PDF 1.4.)
- PDF (the entire document can be submitted rather than individual
fields and values). (defined in PDF 1.4)
AcroForms can keep form field values in external stand-alone files
containing key:value pairs. The external files may use Forms Data Format
(FDF) and XML Forms Data Format (XFDF) files.[44][45][46]
The usage rights (UR) signatures define rights for import form data
files in FDF, XFDF and text (CSV/TSV)
formats, and export form data files in FDF and XFDF formats.[39]
Forms
Data Format (FDF)
The Forms Data Format (FDF) is based on PDF, it uses the same syntax
and has essentially the same file structure, but is much simpler than
PDF, since the body of an FDF document consists of only one required
object. Forms Data Format is defined in the PDF specification (since PDF
1.2). The Forms Data Format can be used when submitting form data to a
server, receiving the response, and incorporating into the interactive
form. It can also be used to export form data to stand-alone files that
can be imported back into the corresponding PDF interactive form.
Beginning in PDF 1.3, FDF can be used to define a container for
annotations that are separate from the PDF document they apply to. FDF
typically encapsulates information such as
X.509
certificates, requests for certificates, directory settings,
timestamp server settings, and embedded PDF files for network
transmission.[46]
The FDF uses the MIME content type application/vnd.fdf, filename
extension .fdf and on Mac OS it uses file type 'FDF '.[39]
Support for importing and exporting FDF stand-alone files is not widely
implemented in free or freeware PDF software. For example, there is no
support in Evince, Okular, KPDF or Sumatra PDF. Import support for
stand-alone FDF files is implemented in Adobe Reader; export and import
support (including saving of FDF data in PDF) is for example implemented
in Foxit Reader and PDF-XChange Viewer Free; saving of FDF data in a PDF
file is also supported in pdftk.
XML
Forms Data Format (XFDF)
XML Forms Data Format (XFDF) is the XML version of Forms Data Format,
but the XFDF implements only a subset of FDF containing forms and
annotations. There are not XFDF equivalents for some entries in the FDF
dictionary - such as the Status, Encoding, JavaScript, Pages keys,
EmbeddedFDFs, Differences and Target. In addition, XFDF does not allow
the spawning, or addition, of new pages based on the given data; as can
be done when using an FDF file. The XFDF specification is referenced
(but not included) in PDF 1.5 specification (and in later versions). It
is described separately in XML Forms Data Format Specification.[45]
The PDF 1.4 specification allowed form submissions in XML format, but
this was replaced by submissions in XFDF format in the PDF 1.5
specification. XFDF conforms to the XML standard. XFDF can be used the
same way as FDF—e.g., form data is submitted to a server, modifications
are made, then sent back and the new form data is imported in an
interactive form. It can also be used to export form data to stand-alone
files that can be imported back into the corresponding PDF interactive
form. A support for importing and exporting FDF stand-alone files is not
widely implemented in free or freeware PDF software. Import of XFDF is
implemented in Adobe Reader 5 and later versions; import and export is
implemented in PDF-XChange Viewer Free; embedding of XFDF data in PDF
form is implemented in pdftk (pdf toolkit).
Adobe XML Forms Architecture (XFA)
In the PDF 1.5 format,
Adobe Systems introduced a new, proprietary format for forms, namely
Adobe XML Forms Architecture (XFA) forms. The XFA 2.02 is referenced in
the PDF 1.5 specification (and also in later versions) but is described
separately in Adobe XML Forms Architecture (XFA) Specification,
which has several versions.[49]
XFA specification is not included in ISO 32000-1 PDF 1.7 and is only
referenced as an external proprietary specification created by Adobe.
XFA was not standardized as an ISO standard. In 2011 the ISO Committee
(TC 171/SC 2/WG 8) urged Adobe Systems to submit the XFA Specification
for standardization.[31]
Adobe XFA Forms are not compatible with AcroForms. Adobe Reader
contains "disabled features" for use of XFA Forms, that activate only
when opening a PDF document that was created using enabling technology
available only from Adobe.[50][51]
The XFA Forms are not compatible with Adobe Reader prior to version 6.
XFA forms can be created and used as PDF files or as XDP (XML
Data Package) files. The format of an XFA resource in PDF is
described by the XML Data Package Specification.[39]
The XDP may be a standalone document or it may in turn be carried inside
a PDF document. XDP provides a mechanism for packaging form components
within a surrounding XML container. An XDP can also package a PDF file,
along with XML form and template data.[49]
PDF may contain XFA (in XDP format), but also XFA may contain PDF.[49]
When the XFA (XML Forms Architecture) grammars used for an XFA form are
moved from one application to another, they must be packaged as an XML
Data Package.[52]
When the PDF and XFA are combined, the result is a form in which each
page of the XFA form overlays a PDF background. This architecture is
sometimes referred to as XFAF (XFA Foreground). The alternative is to
express all of the form, including boilerplate, directly in XFA. It is
sometimes called full XFA.[52]
Starting with PDF 1.5, the text contents of variable text form
fields, as well as markup annotations may include formatting information
(style information). These rich text strings are XML documents that
conform to the rich text conventions specified for the XML Forms
Architecture specification 2.02, which is itself a subset of the XHTML
1.0 specification, augmented with a restricted set of CSS2 style
attributes.[39]
In PDF 1.6, PDF supports the rich text elements and attributes specified
in the XML Forms Architecture (XFA) Specification, 2.2. In PDF 1.7, PDF
supports the rich text elements and attributes specified in the XML
Forms Architecture (XFA) Specification, 2.4[39]
Logical structure and accessibility
A PDF may contain document structure and semantics information to
enable better text extraction and accessibility.
PDF/UA
provides normative text detailing the syntax, features and attributes of
PDF files tagged with complete and accurate information required for
accessibility.
Security and
signatures
A PDF file may be encrypted for security, or digitally signed for
authentication.
The standard security provided by Acrobat PDF consists of two
different methods and two different passwords, user password,
which encrypts the file and prevents opening, and owner password,
which specifies operations that should be restricted even when the
document is decrypted, which can include: printing, copying text and
graphics out of the document, modifying the document, or adding or
modifying text notes and
AcroForm fields. The user password (controls opening) encrypts the
file and requires
password cracking to defeat, with difficulty depending on password
strength and encryption method – it is potentially very secure (assuming
good password and encryption method without known attacks). The owner
password (controls operations) does not encrypt the file, and instead
relies on client software to respect these restrictions, and is not
secure. An "owner password" can be removed by many commonly available
"PDF cracking" software, including some free online services.[53]
Thus, the use restrictions that a document author places on a PDF
document are not secure, and cannot be assured once the file is
distributed; this warning is displayed when applying such restrictions
using Adobe Acrobat software to create or edit PDF files.
Even without removing the password, most freeware or open source PDF
readers ignore the permission "protections" and allow the user to print
or make copy of excerpts of the text as if the document were not limited
by password protection.
Some solutions, like Adobe's LiveCycle Rights Management, are more
robust means of information rights management, which can both restrict
who can open documents, but also reliably enforce permissions in ways
that the standard security handler does not.
Usage rights
Beginning with PDF 1.5, Usage rights (UR) signatures are used to
enable additional interactive features that are not available by default
in a particular PDF viewer application. The signature is used to
validate that the permissions have been granted by a bonafide granting
authority. For example, it can be used to allow a user:[39]
- to save the PDF document along with modified form and/or
annotation data
- import form data files in FDF, XFDF and text (CSV/TSV) formats
- export form data files in FDF and XFDF formats
- submit form data
- instantiate new pages from named page templates
- apply a
digital
signature to existing
digital signature form field
- create, delete, modify, copy, import, export annotations
For example, Adobe Systems grants permissions to enable additional
features in Adobe Reader, using public-key
cryptography. Adobe Reader verifies that the signature uses a
certificate from an Adobe-authorized
certificate authority. The PDF 1.5 specification declares that other PDF
viewer applications are free to use this same mechanism for their own
purposes.[39]
File attachments
|
This
section requires
expansion. (August 2008) |
PDF files can have document-level and page-level file attachments,
which the reader can access and open or save to their local filesystem.
PDF attachments can be added to existing PDF files for example using
pdftk.
Adobe Reader provides support for attachments, and
poppler based readers like
Evince
or Okular
also have some support for document-level attachments.
Metadata
PDF files can contain two types of metadata.[54]
The first is the Document Information Dictionary, a set of key/value
fields such as author, title, subject, creation and update dates. This
is stored in the optional Info trailer of the file. A small set of
fields is defined, and can be extended with additional text values if
required.
Later, in PDF 1.4, support was added for the Metadata Streams, using
the
Extensible Metadata Platform (XMP) to add XML standards-based
extensible metadata as used in other file formats. This allows metadata
to be attached to any stream in the document, such as information about
embedded illustrations, as well as the whole document (attaching to the
document catalog), using an extensible schema.
Subsets
Proper subsets of PDF have been, or are being, standardized under
ISO for several constituencies:
- PDF/X
for the printing and graphic arts as ISO 15930 (working in ISO
TC130)
- PDF/A
for archiving in corporate/government/library/etc environments as
ISO 19005 (work done in ISO TC171)
- PDF/E
for exchange of engineering drawings (work done in ISO TC171)
-
PDF/VT for exchange of variable data and transactional printing
(ISO TC130)
-
PDF/UA for universally accessible PDF technology (work done in
ISO TC171)
Future
ISO
32000-2: Next-generation PDF
Known in PDF syntax terms as "PDF-2.0", ISO 32000-2 will be the first
update to the PDF specification developed entirely within the ISO
Committee process (TC 171 SC 2 WG 8). Publication of ISO 32000-2 is
expected in 2015. Interested parties wishing to participate and resident
in TC 171 Member or Observer countries should contact their country's
Member Body. See ISO's website for more information.
Mars
Adobe was exploring an XML-based next-generation PDF
code-named Mars.[55]
The format of graphic elements of Mars was sometimes described simply
as
SVG,[citation
needed] but according to the version 0.8 draft
specification of November 2007 (§3 Mars SVG Support) the format was
actually merely similar to SVG: it contained both additions to and
subtractions from SVG, so it was in general neither viewable by nor
creatable with standard SVG tools: some things looked noticeably
different between SVG viewers and Mars viewers.
Adobe Systems ceased development of Mars in 2008.[56]
CSS3
The
W3C open standard
CSS3 offers a lot of PDF functionality, see
paged media.
Technical issues
Accessibility
PDF files can be created specifically to be accessible for disabled
people.[57][58][59][60][61]
Current PDF file formats can include tags (XML),
text equivalents, captions, audio descriptions, et cetera. Tagged PDF is
required in the
PDF/A-1a
specification.[62][63]
Some software can automatically produce tagged PDFs, however this
feature is not always enabled by default.[64][65]
Leading
screen readers, including
JAWS,
Window-Eyes, Hal, and
Kurzweil 1000 and 3000 can read tagged PDFs; current versions of the
Acrobat and Acrobat Reader programs can also read PDFs aloud.[66][67][68]
Moreover, tagged PDFs can be re-flowed and magnified for readers with
visual impairments. Problems remain with adding tags to older PDFs and
those that are generated from scanned documents. In these cases,
accessibility tags and re-flowing are unavailable, and must be created
either manually or with
OCR techniques. These processes are inaccessible to some disabled
people.
One of the significant challenges with PDF accessibility is that PDF
documents have three distinct views, which, depending on the document's
creation, can be inconsistent with each other. The three views are (i)
the physical view, (ii) the tags view, and (iii) the content view. The
physical view is displayed and printed (what most people consider a PDF
document). The tags view is what screen readers and other assistive
technologies use to deliver a high-quality navigation and reading
experience to users with disabilities. The content view is based on the
physical order of objects within the PDFs content stream and may be
displayed by software that does not fully support the tags view, such as
the Reflow feature in Adobe's Reader.
PDF/UA,
the International Standard for accessible PDF based on ISO 32000-1 was
published as ISO 14289-1 in 2012, and establishes normative language for
accessible PDF technology.
Viruses and
exploits
PDF attachments carrying viruses were first discovered in 2001. The
virus, named OUTLOOK.PDFWorm or Peachy, uses
Microsoft Outlook to send itself as an attachment to an Adobe PDF
file. It was activated with Adobe Acrobat, but not with Acrobat Reader.[69]
From time to time, new vulnerabilities are discovered[70]
in various versions of Adobe Reader, prompting the company to issue
security fixes. Other PDF readers are also susceptible. One aggravating
factor is that a PDF reader can be configured to start automatically if
a web page has an embedded PDF file, providing a vector for attack. If a
malicious web page contains an infected PDF file that takes advantage of
a vulnerability in the PDF reader, the system may be compromised even if
the browser is secure. Some of these vulnerabilities are a result of the
PDF standard allowing PDF documents to be scripted with JavaScript.
Disabling JavaScript execution in the PDF reader can help mitigate such
future exploits, although it does not protect against exploits in other
parts of the PDF viewing software. Security experts say that JavaScript
is not essential for a PDF reader, and that the security benefit that
comes from disabling JavaScript outweighs any compatibility issues
caused.[71]
One way of avoiding PDF file exploits is to have a local or web service
convert files to another format before viewing.[72]
On March 30, 2010 security researcher Didier Stevens reported an
Adobe Reader and Foxit Reader exploit that runs a malicious executable
if the user allows it to launch when asked.[73]
Usage restrictions and monitoring
PDFs may be
encrypted so that a password is needed to view or edit the contents.
The PDF Reference defines both 40-bit and 128-bit encryption, both
making use of a complex system of
RC4 and
MD5. The PDF
Reference also defines ways that third parties can define their own
encryption systems for PDF.
PDF files may also contain embedded
DRM restrictions that provide further controls that limit copying,
editing or printing. The restrictions on copying, editing, or printing
depend on the reader software to obey them, so the security they provide
is limited.
The PDF Reference has technical details or see
[74]
for an end-user overview. Like HTML files, PDF files may submit
information to a web server. This could be used to track the
IP
address of the client PC, a process known as
phoning home. After update 7.0.5 to Acrobat Reader, the user is
notified "...via a dialogue box that the author of the file is auditing
usage of the file, and be offered the option of continuing."[75]
Through its
LiveCycle Policy Server product, Adobe provides a method to set
security policies on specific documents. This can include requiring a
user to authenticate and limiting the timeframe a document can be
accessed or amount of time a document can be opened while offline. Once
a PDF document is tied to a policy server and a specific policy, that
policy can be changed or revoked by the owner. This controls documents
that are otherwise "in the wild." Each document open and close event can
also be tracked by the policy server. Policy servers can be set up
privately or Adobe offers a public service through Adobe Online
Services. As with other forms of DRM, adherence to these policies and
restrictions may or may not be enforced by the reader software being
used.
Default
display settings
PDF documents can contain display settings, including the page
display layout and zoom level. Adobe Reader uses these settings to
override the user's default settings when opening the document.[76]
The free Adobe Reader cannot remove these settings.
Content
A PDF file is often a combination of
vector graphics, text, and
bitmap graphics. The basic types of content in a PDF are:
- text stored as content streams (i.e., not text)
- vector graphics for illustrations and designs that consist of
shapes and lines
- raster graphics for photographs and other types of image
In later PDF revisions, a PDF document can also support links (inside
document or web page), forms, JavaScript (initially available as plugin
for Acrobat 3.0), or any other types of embedded contents that can be
handled using plug-ins.
PDF 1.6 supports interactive 3D documents embedded in the PDF - 3D
drawings can be embedded using
U3D or
PRC and various other data formats.[77][78]
Two PDF files that look similar on a computer screen may be of very
different sizes. For example, a high resolution raster image takes more
space than a low resolution one. Typically higher resolution is needed
for printing documents than for displaying them on screen. Other things
that may increase the size of a file is embedding full fonts, especially
for Asiatic scripts, and storing text as graphics.
Implementations
PDF-viewing software is generally provided free of charge, and many
versions are available from a variety of sources (List
of PDF software).
There are many software options for creating PDFs, including the PDF
printing capabilities built in to
Mac OS X and most
Linux
distributions,
OpenOffice.org,
Microsoft Office 2007 (if updated to
SP2),[79]
WordPerfect 9,
Scribus,
numerous PDF print drivers for
Microsoft Windows, the
pdfTeX
typesetting system, the
DocBook
PDF tools, applications developed around
Ghostscript and
Adobe Acrobat itself as well as
Adobe FrameMaker.
Google's
online office suite
Google Docs also allows for uploading, and saving to PDF.
Raster image processors (RIPs) are used to convert PDF files into a
raster format suitable for imaging onto paper and other media in
printers, digital production presses and
prepress in a process known as
rasterisation. RIPs capable of processing PDF directly include the
Adobe PDF Print Engine[80]
from
Adobe Systems and Jaws[81]
and the
Harlequin RIP from
Global Graphics.
Editing
There is specialized software for editing PDF files, though the
choices are much more limited and often more expensive than creating and
editing standard editable document formats. Version 0.46 and later of
Inkscape allows PDF editing through an intermediate translation step
involving
Poppler.
Enfocus PitStop Pro, a plugin for Acrobat, allows manual and
automatic editing of PDF files,[82]
while the free Enfocus Browser makes it possible to edit the low-level
structure of a PDF.[83]
See
List of PDF software for a more complete list of PDF editors.
Annotation
Adobe Acrobat is one example of proprietary software that allows the
user to annotate, highlight, and add notes to already created PDF files.
One UNIX application available as
free software (under the
GNU General Public License) is
PDFedit.
Another GPL-licensed application native to the unix environment is
Xournal.
Xournal allows for annotating in different fonts and colours, as well as
a rule for quickly underlining and highlighting lines of text or
paragraphs. Xournal also has a shape recognition tool for squares,
rectangles and circles. In Xournal annotations may be moved, copied and
pasted. The
freeware
Foxit Reader, available for
Microsoft Windows, allows annotating documents. Tracker Software's
PDF-XChange Viewer allows annotations and markups without
restrictions in its freeware alternative.
Apple's
Mac OS X's integrated PDF viewer, Preview, does also enable
annotations as does the freeware
Skim, with the latter supporting interaction with
LaTeX,
SyncTeX, and PDFSync and integration with
BibDesk
reference management software. Freeware
Qiqqa can
create an annotation report that summarizes all the annotations and
notes you have made across your library of PDFs.
For mobile annotation,
iAnnotate PDF (from Branchfire) and
GoodReader (from Aji) allow annotation of PDFs as well as exporting
summaries of the annotations.
There are also
web annotation systems that support annotation in pdf and other
documents formats, e.g.,
A.nnotate,
crocodoc,
WebNotes.
In cases where PDFs are expected to have all of the functionality of
paper documents, ink annotation is required. Some programs that accept
ink input from the mouse may not be responsive enough for handwriting
input on a tablet. Existing solutions on the PC include
PDF Annotator and
Qiqqa.
Other applications and functionalities
Several applications embracing the PDF standard are now available as
an online service including
Scribd
for viewing and storing,
Pdfvue
for online editing, and
Zamzar
for PDF Conversion.
In 1993 the Jaws
RIP from
Global Graphics became the first shipping prepress RIP that
interpreted PDF natively without conversion to another format. The
company released an upgrade to their Harlequin RIP with the same
capability in 1997.[citation
needed]
Agfa-Gevaert introduced and shipped Apogee, the first prepress
workflow system based on PDF, in 1997.
Many commercial offset printers have accepted the submission of
press-ready PDF files as a print source, specifically the PDF/X-1a
subset and variations of the same.[84]
The submission of press-ready PDF files are a replacement for the
problematic need for receiving collected native working files.
PDF was selected as the "native"
metafile format for Mac OS X, replacing the
PICT format
of the earlier
Mac OS.
The imaging model of the
Quartz graphics layer is based on the model common to
Display PostScript and PDF, leading to the nickname Display PDF.
The Preview application can display PDF files, as can version 2.0 and
later of the
Safari web browser. System-level support for PDF allows Mac OS X
applications to create PDF documents automatically, provided they
support the OS-standard printing architecture. The files are then
exported in PDF 1.3 format according to the file header. When taking a
screenshot under Mac OS X versions 10.0 through 10.3, the image was also
captured as a PDF; later versions save screen captures as a
PNG file, though this behaviour can be set back to PDF if desired.
Some desktop printers also support direct PDF printing, which can
interpret PDF data without external help. Currently, all PDF capable
printers also support PostScript, but most PostScript printers do not
support direct PDF printing.
The
Free Software Foundation considers one of their
high priority projects to be "developing a free, high-quality and
fully functional set of libraries and programs that implement the PDF
file format and associated technologies to the ISO 32000 standard."[85][86]
The
GNUpdf library has, however, not been released yet, while
Poppler has enjoyed wider use in applications such as
Evince,
which comes with the
GNOME
desktop environment, which is based on
Xpdf[87][88]
code base. There are also commercial development libraries available as
listed in
List of PDF software.
The
Apache PDFBox project of the
Apache Software Foundation is an open source Java library for
working with PDF documents. PDFBox is licensed under the
Apache License.[89]
See also
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^
Exporting PDF/A for long-term archiving, 2008-01-11
-
^
"Adobe Reader 8 - Read a PDF with Read Out Loud".
Retrieved 2010-04-24.
-
^
Biersdorfer, J.D. (2009-04-10).
"Tip of the Week: Adobe Reader’s ‘Read Aloud’ Feature".
The New York Times.
Retrieved 2010-04-24.
-
^
Accessing PDF documents with assistive technology: A screen
reader user's guide (PDF), Adobe,
retrieved 2010-04-24
-
^
Adobe Forums,
Announcement: PDF Attachment Virus "Peachy", 15 August 2001.
-
^
"Security bulletins and advisories". Adobe.
Retrieved 2010-02-21.
-
^
Steve Gibson - SecurityNow Podcast
-
^
PDFCleaner - PDF Exploit Sanitizer
-
^
PCmag.com blogs
-
^
"Create Adobe PDF Online - Security Settings Help".
Createpdf.adobe.com.
Retrieved 2010-02-21.
-
^
New features and issues addressed in the Acrobat 7.0.5 Update
(Acrobat and Adobe Reader for Windows and Mac OS)
-
^
"Getting Familiar with Adobe Reader > Understanding Preferences".
Retrieved 2009-04-22.
-
^
"3D supported formats". Adobe. 2009-07-14.
Retrieved 2010-02-21.
-
^
"Acrobat 3D Developer Center". Adobe.
Retrieved 2010-02-21.
-
^
"Description of 2007 Microsoft Office Suite Service Pack 2
(SP2)".
Microsoft. Retrieved
2009-05-09.
-
^
Adobe PDF Print Engine 2
-
^
www.globalgraphics.com/products/jaws_rip/
-
^
[1]
-
^
http://www.enfocus.com/product.php?id=4530
-
^
Press-Ready PDF Files "For anyone interested in having their
graphic project commercially printed directly from digital files
or PDFs." (last checked on 2009-02-10).
-
^
Current FSF High Priority Free Software Projects (last
checked on 2009-02-10)
-
^
Goals and Motivations - GNUpdf
-
^
Poppler homepage "Poppler is a PDF rendering library based
on the xpdf-3.0 code base." (last checked on 2009-02-10)
-
^
Xpdf license "Xpdf is licensed under the GNU General Public
License (GPL), version 2 or 3." (last checked on 2012-09-23).
-
^
The Apache PDFBox project . Retrieved 2009-09-19.
Further reading
External links