We use a wiki (specifically the one provided by Trac) for the two reasons you mentioned. Plus, if we really need to we can get the text version of the markup and manipulate it in a text-only environment, too (e.g. as part of svn comments during commit).
A format that can be easily reduced to text-only (non-binary) is definitely a must. Having the ability to upconvert it to a pretty format like a PDF is, for us, not terribly important.
