Help with Book Formatting discussion

I belong to some other forums, and have been sending free digital files to people on them who are sick, injured, or in some other circumstance where they are financially challenged and spending lots of time in bed or on the couch. I recently offered a recent amputee a file, and she says she has a Nexus 7. Doing some research, it seems Google has made it hard to read anything on their tablet unless it is purchased from Google. Any advice for her on reading generic ebook files on her tablet?(

Does an ePub not work? Or is she able to download the Kindle app? I don't anything about the Nexus to be honest, but surely there must be a format that works.