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File type: PDF

What is PDF?

The PDF file extension represents Portable Document Format files. PDF allows documents to be created and viewed in a consistent manner on multiple platforms.

The PDF format has become a standard for distributing documents across the Internet and for creating documents that are intended to be read on a computer.

More on PDF from Wikipedia

Why can't I open PDF files?

In order to open PDF files, you must have a program installed on your computer that is capable of opening them. FileRatings can help you find an appropriate program.

 
Filename extension.pdf
Internet media typeapplication/pdf

application/x-pdf application/x-bzpdf

application/x-gzpdf
Type code'PDF ' (including a single space)
Uniform Type Identifiercom.adobe.pdf
Magic number%PDF
Developed byAdobe Systems
Initial release1993
Latest release1.7
Standard(s)ISO/IEC 32000-1:2008
WebsiteAdobe PDF Reference Archives

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PDF

Portable Document Format (PDF) is a generic computer term. The best-known PDF implementation is Adobe PDF, a file format created by Adobe Systems in 1993 for document exchange. The remainder of this article discusses Adobe PDF exclusively.

Adobe PDF is used for representing two-dimensional documents in a manner independent of the application software, hardware, and operating system. Each Adobe PDF file encapsulates a complete description of a fixed-layout 2D document that includes the text, fonts, images, and 2D vector graphics which compose the documents. Lately, 3D drawings can be embedded to PDF documents with Acrobat 3D using U3D or PRC and various other data formats.

Adobe Systems co-founder John Warnock outlined a system called "Camelot", that evolved into the Portable Document Format (PDF) file-format.

Formerly a proprietary format, PDF was officially released as an open standard on July 1, 2008, and published by the International Organization for Standardization as ISO/IEC 32000-1:2008.

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.

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. PDF is a file format, not a programming language, so that 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:

  • 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 imperative 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 in order 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 issues

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 can automatically produce tagged PDFs, however this feature is not always enabled by default. 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. 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 ISO 32000.

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 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

PDF format attachments carrying viruses were first discovered in 2001. This 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.

From time to time, new vulnerabilities are discovered in various versions of Adobe Reader, prompting the company to issue security fixes. One aggravating factor is that Adobe Reader is by default integrated into browsers, and can be started automatically if the web page has an embedded PDF file, opening up a new vector of attack. If a malicious web page contains an infected PDF file that takes advantage of some vulnerability in Adobe Reader, the system is compromised even if the browser is up-to-date.

On March 30, 2010 security researcher Didier Stevens reported an "exploit" that causes an arbitrary executable to be run when a PDF file is opened, after the user accepts a warning prompt. The exploit works in several different PDF viewers including Adobe Reader and Foxit Reader.

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 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 will be notified "via a dialogue box that the author of the file is auditing usage of the file, and be offered the option of continuing."

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.

Missing PostScript features

Compared to the PostScript format, PDF lacks e.g. the notion of "tray selection"; this can be used to indicate that some pages of a document must be printed on a different type of paper.

Such features are not omissions from the PDF format, whose scope only covers electronic documents. The JDF standard covers such aspects; however, it is a complex standard, which as of 2010 is not widely implemented. This hinders the replacement of PostScript by PDF.

Default display settings

PDF documents can contain display settings, including the page display layout and zoom level. Adobe Reader will use these settings to override the user's default settings when opening the document. The free Adobe Reader cannot remove these settings.

Content

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

  • text stored as such
  • 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.

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.

Standard Type 1 Fonts

There are fourteen typefaces that have a special significance to PDF documents:

  • Times (v3) (in regular, italic, bold, and bold italic)
  • Courier (in regular, oblique, bold and bold oblique)
  • Helvetica (v3) (in regular, oblique, bold and bold oblique)
  • Symbol
  • Zapf Dingbats

These fonts, sometimes referred to as the "base fourteen fonts" should always be present (actually present or a close substitute) and so need not be embedded in a PDF. PDF viewers must know about the metrics of these fonts. Other fonts may be substituted if they are not embedded in a PDF.

Implementations

PDF-viewing software is generally provided free of charge, including versions by Adobe Reader, Foxit Reader, PDF-XChange Viewer, Sorax Reader and others.

There are many software options for creating PDFs, including the PDF printing capabilities built in to Mac OS X and some versions of Linux, the multi-platform OpenOffice.org, Microsoft Office 2007 (if updated to SP2), WordPerfect since version 9, numerous PDF print drivers for Microsoft Windows, the pdfTeX typesetting system, the DocBook PDF tools, applications developed around Ghostscript and Adobe Acrobat itself. Google's online office suite Google Docs also allows for uploading, and saving to the PDF format.

Editing PDFs (structure)

There is also specialized software for editing PDF files, though the choices are much more limited and often expensive. As of version 0.46, Inkscape also allows PDF editing through an intermediate translation step involving Poppler.

Annotating PDFs

Adobe Acrobat is one example of proprietary software that allows the user to annotate, highlight, 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 Linux 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 allows annotating but adds a watermark on each annotated page. The commercial version of the package does not have this limitation. 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.

There are also web annotation systems which allow to annotate pdf and other documents formats, e.g. A.nnotate, WebNotes.

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.

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. 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 Print command. 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; in 10.4 and 10.5 the default behaviour is set to capture as a PNG file, though this behaviour can be set back to PDF if required.

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." 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, at the expense of relying on the GPLv2-licensed Xpdf code base that can't be used by GPLv3 programs.

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.

Use one of the following applications to open PDF files:

  1. InfoFoxit Reader

    License: Free
    Operating system: Win 7, Vista, XP, 2003, 2000

  2. InfoAdobe Reader
  3. InfoEvince