There's two things going on here. It displays as Next because Windows defines the "PageDown" and "Next" keys with the same value. When iTunesKeys converts from this value to the text, it shows as Next because it comes alphabetically before PageDown. But it is still detecting the PageDown key correctly.
The other thing happening is because of the way the hotkeys system works. If you have a command set to PageDown, iTunesKeys "captures" that key and it won't work for anything else. If you've reset/cleared the hotkey bound to PageDown and it doesn't start working right away, restart iTunesKeys and everything should be back to normal.