Add the possibility to add “post-it” like page marker to one or more page, and then the possibility to enable them side-viewing. It’ll be really more user-friendly than the current index function, where you need some clicks to display every time, and makes your experience stressfull and frustrating. Also the auto-level of these page marker would be top, so you can write on the them and have a clean,full-view of what the document contains. Photo attached for example