Ted wrote, quoting me:
In many cases, xhtml is a better format for document interchange.
Horses for courses, of course; and where xhtml may be useful, XML may be even better. If you're not fussed about how the person at the other end will format the layout, then it's probably OK (depending on what kind of document it is). If what you want them to see is precisely what you see, then it can get very hairy.
As this thread has shown, I think, PDF is also about as hairy because of the fragmentation of the standard by its owner. Files look different in each viewer and some viewers don't like some files. At least with xhtml+css, you can specify as much formatting as the viewer supports, but they can nearly always see it all, even if misformatted. With PDF, if the viewer doesn't support all the formatting, you're left with some unhelpful error message and no way to recover the information.