Honestly the only way I've found to make this stuff work is to use something like LaTeX, where the source is a text-based format, and then put it in a real version control system like git, that has proper support for merging and resolving conflicts. Then when the document is correct you compile it to a PDF, and that's what you actually send out (bonus: much smaller file size than something like word).

(Of course, the big downside is that all your collaborators have to know LaTeX, and not use funny macros that the others don't understand)