"the best is paper prototyping"

This was true when I was doing it in grad school in 2000ish as well. I would not be surprised if it was true long before that and will remain so long after today.

I used to think that prototyping on a paper is expensive since paper is non-reusable. Then I proceeded to prototype on an expensive tablet.

Funny looking back at it now.

Adobe XD is a free tool for prototyping, though not Open Source.

Is it useful without the rest of their creative suite? I only ask because Premiere, After Effects, and Photoshop are most effective when used together and obviously designed for that kind of synergy.

Yes it is useful without the rest of the suite. I would also recommend Figma which is also free.

Design of Everyday Things & Don't Make Me Think are both likely to be in your employer's library, or on a co-worker's shelf. Not quite free, but I bet you can find a hard-copy to borrow.

Or in your local public library, which is quite free. :)

I suppose you could use GIMP to prototype, but it would be a massive pain. I use Figma (free; web-based; not open source) for personal projects; if you've used PowerPoint, you can use Figma; the learning curve was very shallow.

Most prototyping tools let you export to PDF on PNG, but they usually aren't going to be open source.

Figma is free, not open source however.

I only know Pencil