Annotations and scrolling stops working sometimes?

I can be going about my business, reading my course literature as usual; and then, suddenly, I can't mark the text anymore. I can't scroll. I can't do anything, and nothing works except restarting the program. This is infuriating as all hell, and no it is not because my pdf-file (it's usually a pdf) isn't OCR:d - because it usually is!

Help?