Unfortunately it is not possible to achieve that since you can only capture keypresses when a specific element is in focus.
This means that your visitors will have to select a certain element before the page down/up keys will work.
Currently there are two elements with keyboard events - TextInput and TextBox
See more below
Just to make it clear - i meant this kind of option like on this site:
not possible ?
Wishlist Page is the new official platform for requesting new features. You can vote, comment, and track the status of the requested features (requests from this page will be migrated to the new Wishlist soon).