I can't find a free/open-source software that password protects my PDFs with AES 256.
pdftk just offers 128-bit strength.. In fact I didn't find any other tool that encrypts PDFs.
What tool may I use to password protect my PDFs, using AES 256
QPDF says it supports 256bit encryption in it's massive manual. I didn't read it all and have never tried it. It looks like it's worth a shot.