If OBS is in the foreground and you have the scenes listbox in focus, page up / page down will move the selection, similar to arrow keys.
xmodmap -e 'keysym Next = KP_6'
xmodmap -e 'keysym Prior = KP_4'
And still disappointing as it conflicts with standard presenters when you DON'T want to change scenes but do something like run a slide show.Moving through a list isn't a hotkey, it's basic standardized UI navigation/interaction.
It would fall under the category of simple user error to have the list selected as the active focus item. You can also use Studio Mode to avoid having these changes reflected immediately in live, without switching focus to the Transition button. Which will, by the way, activate if it has focus and the user hits the enter or space key. Again, a basic UI navigation standard.
Because OBS has a slideshow source for this purpose that works great except it has yet to learn how to pass-thru page-up/page-dn to its own active slideshow function if using a PP remote is desired. NORWII N76 presenter has programmable keys for off-standard slideshow software and solves the problem.Even in PowerPoint at the sheet list the Page Up and Down and arrow keys move through the list. This is standard GUI behaviour. Having unrealistic expecations does not change a bit about that.
But why using OBS Studio as a slide-show machine? Why not Window Capture a (PowerPoint) presentation? When focused this will have the desired arrow keys behaviour.
Thanks, I am fully aware of focus. Been coding in the software industry since 1983. We really need to keep focus in the scenes window and on the slideshow scene for several reasons, but paging from the remote makes that a bit difficult. Standard remotes are mostly hardcoded for Powerpoints page-up, page-down for slides, F5/ESC for Play/Stop, and '.' for show/hide. The NORWII allows its emulated keys to be configured away from those hard-coded in OBS. A workaround for Windows users that only have a PowerPoint remote is Microsoft's free Power Toys that allows remapping page-up/page-dn/F5/Esc to other key sequences like Ctrl-F7 and Ctrl-F8 for example. You do lose those for other functions however as they are globally remapped for all windows applications and have to be manually restored. Other key mapping software offers options to quickly restore the defaults.Having the Image Slide Show to work at the Page Up/Down keys is not the problem. I don't have that brand of presenter device to test with, but setting the actual keyboard keys as hotkeys works fine. So if the presenter device actually sends those keystrokes, it should work.
Your problem apparently is with the focus being somewhere else in OBS Studio that also reacts upon Page Up/Down. To prevent that, try to bring the focus (back) to the Image Slide Show by clicking at it once. That can be either in Program or Preview.