Tapping in the center of the page will show/hide the overlay - you definitely do not have to tap the back button and return to the library screen. You can also adjust the overlay toggle mode under Settings->Touch & Pedal Settings->Overlay Toggle Mode. Also, you can use a three finger tap to enter the annotation mode (it's set by default but can be ajdusted under Settings->Touch & Pedal Settings->Touch Actions), so you don't have to bring up the overlay first. You can also switch this to a two finger tap if you prefer.
Mike
Mike