If you’d be fine with having a docker container running, have a look at Signature PDF.
I’ve been using it to manipulate pdfs in similar ways you described and put it on a pi in my homenetwork (autostarting docker would also work).
They have a tester here.
(Also: french open source project)


I’m running on Astro A50 - used to have it before Linux and reaaaaally liked the Gaming/Communication Hardware adjustable mix. Got it working after hacking some udev rules thanks to this gist.
Years later I am now aware of the above named arctix headset and would probably take this, since the new G-Suite for logitech headsets is shit on phone (phone bc not working on linux).