The option to "favourite" a page can do this, but it would be much better if there was an option to mark a page as a "doubt". Or select a certain area and mark that as a "doubt". Also, if possible, a small caption option should also be there with the doubt and you could see all doubts in one place like the way we can see favourites. This would be very helpful when we get a doubt while revising and have to wait to ask somebody for clarification