Sometimes GoodNotes may stuck loading some PDF files. Then it requires a [force restart](http://support.apple.com/kb/ht5137) to reset. Does it usually happen after opening a certain PDF file/page? If it does, the it is likely the case. You can confirm by force-restarting GoodNotes, opening the pages of a PDF file that may be causing the issue, then checking whether all other pages will become blank afterwards. The PDF export function should still work so you can export the problematic file and view it on your computer instead.