Sometimes you will come across these bad PDF files that either crash unrealBook or make it extremely sluggish. Either it's using up all the memory or just will crash unrealBook. Fortunately they are few and far between. I have one of these and unfortunately there's nothing I can do to stop it from crashing. In this case, it's one page out of many. I've traced it down to that one single page. I don't know if it was generated incorrectly, but there's no way for me to tell if it's bad before loading that single page. In the next version of unrealBook will have a global settings toggle that will allow you to reset all of the preferences of unrealBook just in case.
NEW: Picked up this cool tip from GoodReader's site. Apparently if you open the "bad" PDF in Mac Preview, then save it as a new file the "problems" are fixed. I tried it with my bad PDF and yes, it was fixed!