I mean, your best bet is gonna be a used Kindle DX, and you'll email your PDFs to the kindle with the subject line reading "Convert"

Or get a used iPad or like a cheap fire tablet and then throw LineageOS or something on it. Unfortunately, Amazon is the only company I'm aware of that makes cheap tablets that aren't an "immediately throw in garbage can, do not pass go, do not collect $200" and have a halfway competent display.