Interesting story here about Adobe reader (the thing that opens those wonderful pdf files you get all across the net). Basically there is a critical bug in the software.
Critical bugs in terms of net stuff tends to mean that someone can hack into your computer and do lots of fun stuff. Proof-of-concept code already exists for this vulnerability so its likely that malicious code is already out there. This seems similar to another vulnerability found a while ago.
either way be careful with which ever pdf reader and which pdfs you use.