I'm on Windows VirtualBox use windows guests, seems like Ctrl-Page Up/Page Down don't work in them. Personally I need this for Far manager, it's a console app, but I checked this in gui apps (including firefox) and this don't work either.
Typically the right Control Key is the
I used Keyposé to show the captured keystrokes and it is definitely capturing the Control+PageUp chord in a Windows 8 VM.
Can you provide some additional details, such as the version of VirtualBox installed, the guest operating system version and if you have the guest additions installed?