A new vulnerability has been found in the popular PDF file format. If you open a PDF file that exploits this flaw, it can grab control of your computer. So, as always, it's best to be careful. Whether in email or on the Internet, don't open PDF files unless you know they are from a safe source. 

New PDF Zero-Day Vulnerability

September 20, 2007........PC World

The PDF is a common file format used for documents.