If a file has an extension of .pdf then it most likely using the Portable Document Format (PDF), developed by Adobe Systems, Inc. PDF files maintain the original document formatting for both printing and viewing on a multiple computing platforms, including Windows, UNIX and Mac.
An easy way to view PDF files is to download the Adobe® Acrobat® Reader,which is free software for viewing and printing Adobe Portable Document Format (PDF) files on major hardware and operating system platforms.
PDF Navigation Features
Certain features built into the PDF specification allow authors to expand the usefulness of a document. For example, you can add bookmarks, thumbnails of each page, internal and external links, form fields, article threads, buttons for navigation, notes to annotate information, and views to magnify or reduce a page to fit within the user's computer screen. PDF allows users to magnify up to 800% on screen with no loss of clarity.
PDF Size and Performance
For the Web, smaller files are better since download time depends on file size. PDF files can be optimized to reduce file sizes and can create files up to 80% smaller than their HTML counterparts.
PDF files can be "linearized" to allow the user to start viewing the document before it is fully downloaded, similar to the process used to deliver streaming video over the internet.
PDF can be interwoven seamlessly into most Web sites. This combination of HTML and PDF formats enables authors to serve content in its most appropriate format based on the demands of content versus technology. Also, on most popular Web servers there is no compression, translation, or filtering penalty for serving PDF files.
PDF has a well-established security feature set. Authors of PDF files can prevent users from editing, printing, and/or copying content (text and graphics). Software developers can create their own software to read, create, or modify PDF files without special permission or licensing. The only condition that Adobe insists on is that the document author's PDF security settings be respected.