The behavior you want is bound to the "space" and "backspace" keys.
This was done because the space key is very easy to access when reading a document.
On most laptops, pgup/pgdn are only accessible via key combinations.
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in "Fit width" mode PGDOWN/UP jumps to top of next/previous page instead of showing the rest of current page.
why such behavior was choosen ?
The behavior you want is bound to the "space" and "backspace" keys.
This was done because the space key is very easy to access when reading a document.
On most laptops, pgup/pgdn are only accessible via key combinations.