Word = 4711 ko ; pdf 9730 ko
It depends. Are you using the latest version of doPDF? I ask because in version 6 we introduced a text compression algorithm that reduces the size, so this would be a solution only if you use version 5.
If your word document contains images then the pdf will be larger because doPDF doesn't compress the image - one thing to test would be to change the dpi Resolution in doPDF's Printing Preferences to 72 instead of 300 which is the default one.
P.S. Please do not post in the Announcements forum from now on.