This can be seen/changed in the “Shortcut Mapper”, the area of the Notepad++ software that allows customization of keycombos. Invoke it:
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And change to the “Scintilla commands” tab and do some downscrolling:
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So, as a user you probably don’t care about “Scintilla” or even know what it is – it is an editing component that Notepad++ embeds to get its real work (editing text!) accomplished.
Thus, you’d have to have some intuition about what “SCI_PARADOWN” means, but for the most part this isn’t too difficult.
BTW, you could remap this command to Ctrl+down if you wanted, but that keycombo is tied by default to a different command, so you’d have to remap the other command as well so as to not have a conflict.