We publish a 1000 page textbook and the authors have asked for the entire book in Word form to work on the next edition.
I used the InDesign > Export to PDF X1a with normal prepress settings > In Acrobat Pro 11 Save as Other.... > Word document (under Options, choose retain layout).
The results were pretty good, with pictures and layout fairly well preserved. I'm not sure how it will handle tables.
Hope this helps.
TGD