Everytime I try and send a PDF from Preview on my Mac, it opens Apple's mail programme and tries to send it with that. My default mail application is Outlook (both in Outlook and also in Mail so it's not like the Mac doesn't know that I use Outlook. I've hunted across forums and there are quite a few people who have this problem and yet there doesn't appear to be a solution.
Apparently, it is a "feature". Preview calls Mail.app when asked to send a file (and, to be fair to Apple, the menu option is "Mail Selected PDF Document".