I have discovered that epub documents from the Black Library are quite readable through Google Play Books.
I purchased the Cadia Codex supplement for Warhammer 40K as a test, to see if these were worth buying, and if they didn’t work on my tablet, I was actually considering getting a new or refurbished iPad in order for me to read these things.
Before trying Play Books, I had downloaded a few ‘free’ epub3-format readers, and all of them failed processing the text in the document, in one way or another.
I may purchase digital copies of existing rules.