I have a user who edits PDF's downloaded from a web browser. When she first opens the document she scrolls through all the pages to make sure that all the required information is there.
Once she has done that, when she scrolls back to the top to begin reviewing the information, Most if not all pages randomly shrink down in size to tiny dots. I've been able to recreate this easily just as described, and cannot find any issue with her zoom settings, view settings, mouse, or keyboard. (Thought maybe it was throwing a ctrl signal while she was scrolling causing random zooming.)