| Portable Document Format |
Article Index for Portable |
Website Links For Portable |
Information AboutPortable Document Format |
| CATEGORIES ABOUT PORTABLE DOCUMENT FORMAT | |
| adobe systems | |
| graphics file formats | |
| open formats | |
| page description languages | |
| vector graphics | |
| computer file formats | |
| digital press | |
The ''Portable Document Format'' ('''PDF''') is the File Format created by Adobe Systems in 1993 for document exchange. PDF is used for representing two-dimensional documents in a device-independent and Display Resolution -independent fixed-layout document format. Each PDF file encapsulates a complete description of a 2-D document (and, with Acrobat 3-D, embedded 3-D documents) that includes the text, fonts, images, and 2-D Vector Graphics that compose the document. PDF is an Open Standard , and is now being prepared for submission as an ISO standard. HISTORY When the PDF first came out in the early 1990s, its general adoption was slow.1 At that time, the PDF-creation tools ( Acrobat ) and the viewing and printing software had to be bought. Early versions of PDF had no support for external hyperlinks, reducing its usefulness on the World Wide Web ; the additional size of the PDF document compared to plain text meant significantly longer download times over the slower Modem s common at the time, and rendering the files was slow on less powerful machines. Additionally, there were competing formats such as Envoy , Common Ground Digital Paper and even Adobe's own PostScript format (.ps); in those early years, the PDF file was mainly popular in Desktop Publishing Workflow . Adobe soon started free distribution of the Acrobat Reader (now Adobe Reader) program, and continued supporting the original PDF, which eventually became the De Facto standard for printable documents. The PDF file format has changed several times, 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), corresponding to Acrobat releases 1.0 to 8.0. TECHNOLOGY Anyone may create applications that 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.http://partners.adobe.com/public/developer/support/topic_legal_notices.html The PDF combines three technologies:
PostScript PostScript is a Page Description Language run in an Interpreter to generate an image, a process requiring many resources. PDF is a file format, not a programming language, i.e. flow control commands such as if and loop are removed, 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:
Accessibility PDF files can be created specifically to be accessible for disabled people. Current PDF file formats can include tags ( XML ), text equivalents, captions, audio descriptions, et cetera). Some software, such as Adobe InDesign , can automatically produce tagged PDFs. Leading Screen Reader s, including JAWS , Window-Eyes , and Hal, can read tagged PDFs; current versions of the Acrobat and Acrobat Reader programs can also read PDFs aloud. 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. 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. One of the major problems 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 read (useful for people with poor eyesight). The content view is displayed when the document is re-flowed to Acrobat (useful for people with mobility disability). For a PDF document to be accessible, the three views must be consistent with each other. Security In 2001, PDF format attachments carrying viruses were first discovered. Virus researchers found that the PDF file viruses activated with Adobe Acrobat, but not with Acrobat Reader. As with all file formats, caution is advised. An up-to-date antivirus program is paramount. 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 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. Printable documents especially might be saved instead as bitmaps and subject to OCR. The PDF Reference has technical details or see {Link without Title} 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. Subsets Proper subsets of PDF have been, or are being, standardized under ISO for several constituencies:
|
|
|