PDF stands for Portable Document Format.
It is a file format that has captured all the elements of a printed document as an electronic image that you can view, navigate, print, or forward to someone else. PDF files are created using Adobe Acrobat , Acrobat Capture, or similar products. Modern versions of Microsoft Office allow you to save files as PDF. To view and use the files, you need the free Acrobat Reader, which you can easily download. Once you've downloaded the Reader, it will start automatically whenever you want to look at a PDF file.
The PDF is intended for reading on the computer or printing, it is very unsuitable as a source of information on a web site as it cannot be navigated online.