I find the whole class of anti-convulsant mood stabilizers very interesting. They are also sometimes prescribed as migraine preventative medications.

It’s fascinating how these compounds have these multiple indications. It raises question to how the underlying mechanisms of these conditions may interact.

Bollard

Very cat

Won’t qum

I have personally used DrawboardPDF on Windows tablets because it came with one that I bought, but it costs money and there may be better options out there. Nonetheless, it’s probably worth checking out.

Aside from that, I would probably consider looking at Xournal++. It’s technically a note taking program, so probably not quite what you’re looking for, but it handles PDFs and it seems to work well with a stylus. I’ve never used it to any significant extent, however. It’s also free, open source, and cross platform which are major pluses.