Whenever the information can be adequately conveyed in plain text, that
should be the first choice since it can be viewed by all email programs,
If that's not possible, then my order of preference would be
2) pdf (works very well on Unix, Mac and Windows)
3) word processor files, as an absolutely last resort.
All ISPs should include the Acrobat Reader with the basic bundle of
sofware they give the subscribers, or at least have a copy on a local
server, since it is free and will save them bandwidth!