I would assume it'd also work with literal Page Up/Down keys, however that is exposed for keyboards when using Bluetooth. The important part is having that on the phone itself, placed such that when you hold it with one hand, the thumb is conveniently placed on the page down key, so that one only needs to push slightly to flip to the next page. That's what makes Android phones so convenient for reading over long periods of time compared to iPhones, where you have to lift the thumb and tap the screen for every page.