My book's editor sends me PDF files with comments on them (like this, but with much less red ink now). For "technical" reasons, I use Lady Mens' computer (a Windows box) to work on those, and because Adobe Acrobat reader is on that, I used that. Until yesterday. Upon opening the largest chapter, the machine crawled to an almost standstill, and the culprit was quickly found: This same screen shot shows the same PDF open in the free-ish Foxit Reader; quite a difference! Acrobat Reader really sucks, but there are alternatives on Windows.