PDF

Portable Document Format (PDF)
	File:PDF.png
	File:Acroread.png Adobe Acrobat Reader displaying a PDF in Debian GNU/Linux
Filename extension	.pdf
Internet media type	application/pdf
Type code	'PDF ' (including a single space)
Uniform Type Identifier (UTI)	com.adobe.pdf
Magic number	%PDF
Developed by	Adobe Systems

Portable Document Format (PDF), commonly mistaken for "Printable Document Format", is an open file format created by Adobe Systems in 1993 and is now being prepared for submission as an ISO standard^[1]. It is for representing two-dimensional documents in a device independent and resolution independent fixed-layout document format. Each PDF file encapsulates a complete description of a 2D document (and, with the advent of Acrobat 3D, embedded 3D documents) that includes the text, fonts, images, and 2D vector graphics that compose the document. PDF files do not encode information that is specific to the application software, hardware, or operating system used to create or view the document. This feature ensures that a valid PDF will render exactly the same regardless of its origin or destination (but depending on font availability when fonts are not encapsulated in the file).

PDF files are most appropriately used to encode the exact look of a document in a device-independent way. While PDF can describe very simple one page documents, it may also be used for many pages, complex documents that use a variety of fonts, graphics, colors, and images.

History

When PDF first came out in the early 1990s, it was slow to catch on. At the time, not only did the only PDF creation tools of the time (Acrobat) cost money, but so did the software to view and print PDF files. Early versions of PDF had no support for external hyperlinks, reducing its usefulness on the web. Additionally, there were competing formats such as Envoy, Common Ground Digital Paper and even Adobe's own PostScript format (.ps). Adobe soon started distributing the Acrobat Reader (now Adobe Reader) program at no cost, and continued to support PDF through its slow multi-year ramp-up. PDF eventually became the de facto standard for printable documents.

The PDF file format has undergone several changes, generally as new versions of Adobe Acrobat have been released. There have been eight versions of PDF, 1.0 (1993), 1.1 (1994), 1.2 (1996), 1.3 (1999), 1.4 (2001), 1.5 (2003), 1.6 (2005), and 1.7 (2006). The format is not fully forward-compatible, and older reader software may not be able to open newer files.^{[citation needed]}

Technology

Anyone may create applications that read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds a number of patents relating to PDF but licenses them on a royalty-free basis for use in developing software that complies with its PDF specification.^[2]

PDF is primarily the combination of three technologies:

A sub-set 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.

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/UA for universally accessible PDF files

PostScript

PostScript is a page description language that is run in an interpreter to generate an image. This process requires a fair amount of resources.

PDF is a file format instead of a programming language and for that reason it doesn't need to be interpreted. For instance, flow control commands like if and loop are removed, while graphics commands such as lineto remain.

That means that the process of turning PDF back into a graphic is a matter of simply reading the description, rather than running a program in the PostScript interpreter. However, the entire PostScript world in terms of fonts, layout and measurement remains intact.

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 the document references are also collected; and finally everything is compressed into a single file.

As a document format, PDF has several advantages over PostScript:

Already interpreted: PDF contains already-interpreted results of the PostScript source code, so it is less computation-intensive and faster to open, and there is a more direct correspondence between changes to items in the PDF page description and changes to the resulting appearance of the page.
Object transparency: PDF (starting from version 1.4) supports true transparency while PostScript does not.
Font substitution: If displayed with Adobe Reader, a font-substitution strategy ensures the document will be readable even if the end-user does not have the "proper" fonts installed. PDF also allows font embedding to ensure that the "proper" fonts are displayed. While this is possible with PostScript, such files cannot normally be distributed freely because of font licensing agreements.
Independent pages: PostScript is an imperative programming language (with an implicit global state), so instructions accompanying the description of one page can affect the appearance of any following page. All the preceding pages must therefore be processed in order to determine the correct appearance of any given page. Each page in a PDF document is unaffected by any others.

Accessibility

PDF files that are accessible to people with disabilities can be created. Current PDF file formats can include tags (essentially XML), text equivalents, captions and audio descriptions, and other accessibility features. Some software, such as Adobe InDesign, can output tagged PDFs automatically. Leading screen readers, including Jaws, Window-Eyes, and Hal, can read tagged PDFs; current versions of the Acrobat and Acrobat Reader programs can also read PDFs out loud. Moreover, tagged PDFs can be reflowed and zoomed for low-vision readers.

However, many problems remain, not least of which is the difficulty in adding tags to existing or "legacy" PDFs; for example, if PDFs are generated from scanned documents, accessibility tags and reflowing are unavailable and must be created either by hand or using OCR techniques. Also, these processes themselves are often inaccessible to some people with disabilities. Nonetheless, well-made PDFs can be a valid choice as long-term accessible documents. PDF/UA, the PDF/Universal Accessibility Committee, an activity of AIIM, is working on a specification for PDF accessibility based on the PDF 1.6 specification.

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 in which third parties can define their own encryption systems for use in PDF.

PDF files may also contain embedded digital rights management 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 very limited. Documents that are printable can be printed by using Microsoft Office Document Image Writer to create .mdi files. Image Writer has an OCR to Microsoft Word conversion option that seems to preserve tables and yields files that can be edited.

The PDF Reference has technical details or see [1] 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.

Through their 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 time frame 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.

Mars

According to a 7 December 2006 Government Computer News blog, Joab Jackson writes that Adobe is exploring an XML-based next-generation PDF code named Mars: http://www.gcn.com/blogs/tech/42740.html

Adobe has published information about the Mars file format at http://www.adobe.com/go/mars and at http://labs.adobe.com/wiki/index.php/Mars.

Content

A PDF file is often a combination of vector graphics, text, and raster graphics. The basic types on content in a PDF are:

text stored as such that can be easily copied to another program
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 3D documents embedded in the PDF that can be interacted with.

Two PDF files which 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

Readers for many platforms are available, such as Xpdf, Foxit, and Adobe Reader; there are also front-ends for many platforms to Ghostscript. PDF readers are generally free. There are many software options for creating PDFs, including the PDF printing capability built in to Mac OS X, the multi-platform OpenOffice.org, numerous PDF print drivers for Microsoft Windows, and Adobe Acrobat itself. There is also specialized software for editing PDF 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 of Mac OS X is based on the model common to Display PostScript and PDF, and is sometimes somewhat confusingly referred to as Display PDF. Due to OS support, all OS X applications can create PDF documents automatically as long as they support the "print" command. When taking a screenshot under Mac OS X versions 10.0-10.3 the image was also captured as a PDF; in 10.4 the default behaviour is set to capture as a PNG file, though this behaviour can be set back to PDF if required.

With the increasing popularity of PDF, some 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.

References

^ Adobe Systems, Inc. (2007-01-29). "Adobe to Release PDF for Industry Standardization". Retrieved 2007-01-31.
^ http://partners.adobe.com/public/developer/support/topic_legal_notices.html

External links

PDF Reference
PDF 1.7 reference, released Jan. 29, 2007, (PDF)
PDF Reference Book - official reference to the PDF file format from Adobe, also available as a book describing PDF 1.6 (ISBN 0-321-30474-8) .
Template:PDFlink — A white paper from PDF Tools AG with an introduction into what PDF is and its strengths and weaknesses.
Template:PDFlink — details of the four possible formats generated by Adobe Acrobat Capture 3.0. The title is misleading as it does not describe four different formats of PDF, but rather four different options for PDF creation in one program. The link may be worthy of note because this informal guide has led to the widespread myth that these are, in fact, four (and the only four) different types of PDF file.
Adobe: PostScript vs. PDF
History of PDF at prepressure.com
Optimizing PDFs for User Search Tao Fujimoto
The Camelot Paper — the paper in which John Warnock outlined the project that created PDF
AIIM — Information about PDF/E specification for engineering
AIIM — Information about PDF/UA specification for accessible documents
Mars — Further information about the Adobe Mars Format

[1] Adobe Systems, Inc. (2007-01-29). "Adobe to Release PDF for Industry Standardization". Retrieved 2007-01-31.

[2] ttp://partners.adobe.com/public/developer/support/topic_legal_notices.html

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