I think my lawyer would be concerned to hear this. When I asked him why he always sends letters etc as .pdf's he said that it was so they can't be modifed and then sent on, thus misrepresenting what was in the original. I've also noticed that accountants and IFA's often do the same.
Unlike me, he's quite a high-tech sort of person (I'mn a Luddite) so seriously, just how easy is it to modify/overwrite a .pdf?