I tried to do the same as you did (on Windows 7); two large PDF files, commenting on one, then opening the 2nd and commenting - no problem.
So I don't know what is causing your issue, sorry!
I tried to do the same as you did (on Windows 7); two large PDF files, commenting on one, then opening the 2nd and commenting - no problem.
So I don't know what is causing your issue, sorry!