The PDF book idea (for me where frequent big downloads are not an issue; some have had trouble with large attachments in the past) seems heartening and fun :)

Regarding alternatives to PDF, a clear format is plaintext. This is easier to read and trust from a text-only environment, and can be smaller and easier to transfer. There are tools to convert pdfs or epubs to plaintext, but often the output needs a lot of hand-massaging to look nice.

There are some good examples of plaintext books at the old project gutenberg website.

However, I infer doing that conversion could add a lot of work to something that otherwise could be an easy share, so I don't personally have an opinion on it one way or the other.