A PDF (portable document format) is a file often used to transfer documents that need to look official. One advantage of PDF is probably that unlike Microsoft Word files (.doc files), PDFs cannot contain virus-like macros that can cause problems for your computer!
You will need a program called "Adobe Acrobat" to view a PDF file. It is produced by Adobe Systems and can be downloaded for free.
PDF is used by many websites and companies. For example, many of the forms available on the main 4-H page are in PDF format.