Well avast doesn't differentiate between files, it just scans content and alert to infection, are you getting any alerts ?
If so What ?
What are the symptoms, can't search for pdfs (errors, redirects, etc.), can't open pdfs on-line (again errors, etc.) ?
Personally I wouldn't recommend opening pdfs on-line, download and view off-line.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What is your firewall ?
What other security based software do you have installed ?
