This is a simple answer.
I work in Executive Search Recruitment and we always ask for CV's / Resume's to be in word format. This is simply because we apply each resume onto our own letterhead with the company logo etc in the background. It also allows the recruitment company to remove any of your contact details if your CV is being sent to an employer to review. If it is in PDF and still contains your contact details there is a risk that the company will contact the candidate directly, meaning that the recruiter that has introduced you is cut out of the loop and receives no commission.
I get people constantly asking this question and it's hilarious what some people assume you're doing it for. But the fact is that the recruiter needs to remove your contact details to stop companies contacting you directly and avoiding paying a recruitment fee.