I use Xournal++, too. Note that the original Xournal is no longer maintained, but Xournal++ is.
It’s supposed to be a clone of Windows Journal - the precursor to OneNote. It’s very good at exporting to and annotating PDFs, and I use it for all my classwork. Windows Journal worked great for me back in the day, and Xournal++ continues to do so today.
All that said, I’m saving this post so I can try out some of the alternatives listed here in the future.