What are .pdf files?

If you've been using the internet for any amount of time, you've probably run across pdf files before, amybe not even known they are called pdf files! They're Adobe Acrobat files.

Normally, you download them and use Adobe Acrobat Reader to read them. You can get Adobe Acrobat Reader for free at Adobe's web site:
http://www.adobe.com/products/acrobat/readstep.html (Make sure you un-check the "McAfee Security Scan Plus" check-box if you don't want the tagalong software).

People use PDF files for everything. The most common use seems to be documents (both interactive and regular), but I've also seen PDF files used for maps, catalogs, presentations, and even e-books. Additionally, I've seen it used for non-web applications as well, including stuff like software help files.