Methinks that you get to write a WYSIWYG text editor for that client.
Might be easier for the doc owner to do their editing on their own
software and then uploading the final doc.
If that needs editing - - - well then someone downloads the doc shifts
it back to a word processor file from pdf
does the editing changes back to a pdf and then uploads the now edited
file. That process is fraught with
possible gotchas - - - I can see why you might want such an
application - - - don't know of one though.
Maybe someone else has a better idea?
HTH