Fixed horizontal scroll
I often find myself annoyed when I read zoomed documents because when I move to the next page I need to be careful not to move it horizontally as my zoom makes text fit exactly screen borders.
So, here is what I suggest. Add a button somewhere on the action bar/toolbar (android/ios) so that is locks horizontal scroll at the current position.
This would be helpful for people like me, who reads on mobile devices. Because very often books have huge margins which take a lot of so valuable space on small screen.
With such feature users may zoom documents so that the text fits screen borders, then lock the horizontal scroll and enjoy reading!