It would be so cool if you could add sticky mage markers (see attachment) to your notebooks and PDF documents – just like in a "real" notebook. If you "click" on a page marker, you jump to that section. But the difference is, this does not have to be a structure point or something.

Also, I cannot use bookmarks, because I cannot arrange those in several categories which I would definitely need.

Also, this would be quite handy.