iPad app Goodreader includes the color highlight pens, but one thing it has that I especially like and of which to my knowledge isn't supported yet in any dedicated music display app is the ability to either universally (all pages), or selectively crop "all" left or "all" right pages of a large music book, such as a large 1000+ page PDF fakebook. Not sure if if UnRealBook or any other dedicated music display app has finally incorporated this very useful feature yet or not. Anybody know? Though I own UnRealBook, Until it includes the above GoodReader features, GoodReader remains my prefered music display app.