As pianist I found very handy to turn pages via midi (foot controller / breath controller - hands free). I referring to midi cc messages (connection via alsa or jack-audio-connection-kit or osc [open sound control]). Turn page can respond to user-defined 'midi cc' values. I suggest for defaults cc 16 for next page and 17 for previous page. (see p. 6 http://www.midi.org/techspecs/midi_chart-v2.pdf ) Midi data usually (or accidentally by the player) are continuous, so the midi data stream must be evaluated. You can simulate midi data cc's with programs 'vmpk' and 'gmidimonitor'. You can connect the midi stream between applications with cli 'aconnect' or via GUI with 'qjackctl'. (excuse my English)
I do not think this is Okular-specific. There could be a software/driver generating regular XI input events from this devices, so that all applications could set up shortcuts using this device.
Agreed with Christoph, hacking this support directly into Okular doesn't make any sense, this should be a system level service/component/app
I was expecting this answer. Fair enough. Thanks for the time to answer this :)