You can create your own keyboard shortcuts and if you don't want Ctrl+PageDown to do anything other than go to the next page then you should assign that as your preferred shortcut.

File > Options > Customize Ribbon > Keyboard shortcuts [Customize]
Category = All Commands
Command = GoToNextPage
Press New Keyboard Short = Ctrl+PageDown
Save Changes in = Normal.dotm

Personally, I generally don't like to override the default keyboard shortcuts (to avoid getting in to habits that don't work on other machines or ruining shortcuts that other people use) so I go looking for unassigned options. If it were me I would assign this to Ctrl+Shift+PageDown which is available. And while I was at it I would also set the pageup (GoToPreviousPage) to be consistent.

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Andrew Lockton
Chrysalis Design, Melbourne Australia