There is a current zero day exploit using PDF files right now. It seems to be a targeted attack against defense contractors at the moment, but who knows.
http://www.adobe.com/support/security/advisories/apsa11-04.html
and
http://www.computerworld.com/s/article/9222454/Hackers_exploit_Adobe_Reader_zero_day_may_be_targeting_defense_contractors
Perhaps Avast has picked up on this in some way???