I had a quick look at this, and it isn't obvious to me how to do this (and I'm the developer!).
However, on reflection, I don't think this is a good idea anyway. The export to Word facility is really intended as a temporary facility to enable Teachers to do a once-off export of a Moodle Book to Microsoft Word, so that they can edit it in Word. It is not intended for Students to treat it as a complete and true copy of the Book content. For example, images are not exported in a way that is directly visible to them.
If a Teacher wants to make a printable version of a Book available, then I recommend that they export the Book to Word, prepare it (format it with appropriate headers and footers, re-instate images, etc.), and finally save the Word document as a PDF file, and upload that as a separate file beside the Book.
You get the best PDF file if you use Words' "Save As" command, select "PDF (*.pdf)" in the "Save as type:" field, click the "Options" button, and set the options as shown below: