Change one-half page at a time
When viewing a PDF music score and playing along with it, it is desirable to change one-half page at a time instead of one full page. Often, foot pedal controls are used which simulate or emulate the keyboard. So most convenient would be for PgUp and PgDn keys to change page by one-half.
My correction: What is actually desired is to show 1 page at a time (say page 1). Then change to show the top half of the next page (page 2) while keeping the bottom half of current page (page 1) showing. Then next change is to keep that top half of page 2 still at top and change the bottom half from page 1 to page 2. Now top and bottom of page 2 is showing. Repeat this.
What happens is the musician plays down from top of page 1. Once playing past half of page 1, a key is pressed (or a foot pedal is pressed) to change the upper half to the next page so that once the bottom f page 1 finishes being played, the next page (2) is already existing at the top half. The eyes can quickly move the begin playing from the top of the page again.
Then once playing from the top of the page has started again (i.e. page 2), the hey or pedal is pressed again and the bottom half changes to show the next page (page 2), so that the player can play from that top of page 2 all the way down to the bottom of page 2. Note though that once played through the top half of page 2, the key/pedal would be pressed again to bring the top half of page 3 in.