Hi,
   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.