I have two android tablets, one 7" to read small books, and one 13" to read US Letter format books, I took the cheapest ones I found, disabled Google Play and installed F-Droid to install FOSS readers, and it just works perfectly. You really don't need anything specific to just read text, you just want to make sure that you can display an entire page on your screen in a size you're comfortable reading, otherwise PDFs becomes quickly insufferable.