Portable Document File
From Wikicpa
Portable Document Format (PDF) is an open standard file format, proprietary to Adobe Systems, for representing two dimensional documents in a device independent and resolution independent 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. Importantly, PDF files do not encode information that is specific to the application software, hardware, or operating syste] 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. PDF is also an open standard in the sense that anyone may create applications that read and write PDF files without having to pay royalties to Adobe Systems; Adobe has a number of patents relating to the PDF format, but licenses them on a royalty-free basis for use in developing software that complies with the PDF specification.[1]
PDF files are most appropriately used to encode the exact look of a document in a device-independent way. While the PDF format can describe very simple one page documents, it may also be used for many pages, complex documents that use a variety of different fonts, graphics, colors, and images.
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Technology
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, and
- a structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.
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 Adobe Acrobat cost money, but so did the software to view and print PDF files. Early versions of the PDF format had no support for external hyperlinks, reducing its usefulness on the web. Additionally, there were competing formats such as Envoy (WordPerfect), Common Ground Digital Paper, DjV] and even Adobe's own PostScript file format (.ps). Adobe started distributing the Acrobat Reader (now Adobe Reader) program at no cost, and continued to support PDF through its slow multi-year ramp-up. Competing formats eventually died out, and PDF became a well-accepted standard.
In 2005 Microsoft presented a competing format named XML Paper Specification (XPS). XPS is based on XAML, and is distributed along a royalty-free license. XPS support is scheduled to be included in Microsoft Windows Windows Vista.
Macintosh
PDF was selected as the "native" metafile format for Mac OS X, replacing the PICT format of the earlier Mac O]. The imaging model of the Quartz (graphics layer) 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.
PDF and accessibility
PDF can be accessible to people with disabilities. 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 Optical character recognition techniques. Also, these processes themselves are often inaccessible to the people who would benefit from them. Nonetheless, well-made PDFs can be a valid choice as long-term accessible documents. (Work is being done on a PDF variant based on PDF 1.4. The PDF/A or PDF-Archive is specifically scaled down for archival purposes.)
Microsoft Word documents can be converted into accessible PDFs, but only if the Word document is written with accessibility in mind - for example, using styles, correct paragraph mark-up and "alt" (alternative) text for images, and so on.
PDF on the Web
Documents described in markup languages such as HTML/XHTML delegate responsibility for many display decisions to the renderer. This means that an XHTML document can render quite differently across various web browser platforms. While the end user experience of an XHTML document can vary significantly depending on browser, platform, and screen resolution, a PDF file can be reasonably expected to look exactly the same to every viewer. The desire for greater control over user experience has led many authors to use the PDF format to publish online content. This is particularly true for order forms, catalogues, brochures, and other documents which are primarily formatted for printing. The ubiquity of Adobe Reader and wide corporate availability of easy to use WYSIWYG PDF authoring have further enticed many (mostly corporate) web authors to publish a wider variety of information as PDF.
Critics of this practice cite several reasons for avoiding it. The major one is that the inflexibility of PDF rendering makes it difficult to read on screen: it does not adapt to the window size nor the reader's preferred font size and font family, as classic XHTML web page does. PDF files tend to be significantly larger than XHTML/Scalable Vector Graphics files presenting the same information, making it difficult or impossible for users with low-bandwidth connections to view them. Adobe Reader, the de facto standard PDF viewer, has historically been slow to start and caused browser instability, particularly when run alongside other browser plugins (Adobe Reader 7 addressed many of these concerns, but is not available under Windows 98/ME). Adobe Reader is also unavailable in current versions on many alternative operating systems and is distributed under a proprietary license unacceptable to some users. During each major release of Adobe (Acrobat) Reader, the installer package gets significantly larger to support extra features, but users are left without means to selectively install components.
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 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 [2] 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 phone home.
PDF hardware
With 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 not the other way around.
External links
- White Paper: PDF Primer - A white paper from PDF Tools AG with an introduction into what PDF is and its strengths and weaknesses.
- Adobe Publication: The Four Flavors of Adobe PDF for Paper-based Documents (PDF) - 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 lead to the widespread myth that these are, in fact, four, and the only four, different types of PDF file.
Format information
- PDF Specification, also available as a book describing PDF 1.6 (ISBN 0321304748)
- Adobe: PostScript vs. PDF
- History of PDF at prepressure.com
- The Camelot Paper — the paper in which John Warnock outlined the project that created PDF
