Friend instaled avst for me. And now I can’t look for PDF files in internet. But I need them for my studies. How to change thees setings? Please hellp me…

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 ?

If you are using Adobe reader open it up click help detect repair, then update.
The basic is 7 I think