As already noted in the question, you seem to want to rearrange the page down to a different key (due to the limited keyboard).
To determine which command is mapped, to open the keyboard settings dialog and enter the current key in the short key text field, and then look at what it is currently mapped to.
Unfortunately, this does not work for the page down (the control page down works, but this does not help here).
Entering the "page" in the command filter: Edit.PageDown displayed in the text editor in the "Down down". Match this command with another shortcut.
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