The submit by email is hardcoded to send an xml file (Note that by default Reader can only send the xml file). If you want to send the PDF then you will have to drop a button object from the standard palette, set the Control Type to Submit, and on the Submit tab use the mailto protocol as the Submit To URL parameter. Just below that there is a Submit dropdown where you can pick PDF as your submission. Now if your users are using Reader you will have to Reader Extend the file before they can use your button.
Hope that helps