When trying to annotate using a pen Okular doesn't differentiate between touch and pen events; That means in order to scroll with the touch screen one has to turn off the annotation tool first. As a solution I think there should be an option or a button to disable drawing with touch.
I never used a touchscreen, but scrolling with two fingers should work fine. Did you try that? AFAIK two finger gestures are sent as “pinch” gesture event first, and then Okular just needs to process and accept this gesture event. I don’t know how comfortable that is, since it is also used for zooming.
Can reproduce. Touch and pen are indeed different inputs, and were Okular to use QtQuick for the UI, this is something we'd be able to easily do. As it, I guess we need to handle it manually.
The two finger gesture does not work if an annotation is enabled. It paints with the first finger and only zooms. Either way though a two finger swipe is not really very usable exactly because it also zooms.
I can confirm Zamundaaa's comments in Windows too, with the current nightly build (as of 2021-09-18, v. 21.11.70) and the stable version (20.12.2): when annotation is enabled, scrolling with two fingers does not work (it zooms and it paints). This makes it cumbersome (need to turn annotation on/off) to use Okular on tablet devices for commenting/annotating.