Don’t browsers allow you to do form fillable these days? I swear i just filled one with firefox the other day. Maybe that’s too limited?

For combining pdfs, pdftk from the command line is my goto. The command line interface for it isn’t too complicated.

Linux mint or fedora are great options. I’d probably recommend mint as usually geared more to those switching from windows and the forums and communities might be more helpful for beginners.

I’d avoid ubuntu these days. While not horrendous, i’ve run into enough annoyances with their snap package manager that there’s just no reason to learn this additional thing for a beginner.