I downloaded a pdf book that has hundreds of pages and realized that I'm going to have to go through all these pages if i want to find this one again.
A cool, useful idea to solve this could be having the ability to color code pages or sort of bookmark pages so you can click a button of some sort and then it gives you a selection of pages that are bookmarked, choose the page you want and it brings you to it.

I think this could be a feature that will be SOOOO useful for people with large notebooks like me:)

p.s. this would be a good feature for ios version aswell.