When I was distributing lyrics to a singing group, some of whom printed them and some of whom sung from an iPad - and I from my Android tablet - there was one format that worked for everybody - PDF. In fact I experimented and found that a pdf inside a zip file could be displayed - without unpacking it - and resized to fit the screen, on both Android and iPad.

As for text files, this 10" Samsung Android tablet does not actually recognise a plain text file. If I create or download one it won't open it in anything. Unless I add .txt on the end - which is an ancient Microsoft decoration which even Windows doesn't need any more - then it recognizes it as a text file, but offers no built-in app to read it.

Same seems to be true on an iPad. If I email a plain text file to my wife's iPad, it won't open it. If she had a Word Processor installed that would probably open it - but it wouldn't be a text file any more.