If you drag a one page pdf file into Apple Mail it usually appears as an inline image. If you want the file to just appear as an attached document icon just control-click on the image in your mail message and select View as Icon from the pop-up menu. You can go back to the image style by control-clicking and selecting View in Place. This only works with one-page PDF files.
Sending the PDF as an icon seems to prevent problems I have had with some Windows' recipients seeing only the image and not getting the PDF attachment. You may also want, or not want, the image inline for aesthetic reasons.
[robg adds: In all my time with Mail, I'd written off the View in Place menu item, as it was always grayed out -- but now I know that's because I'd never tried it with a one-page PDF.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070307044004964