1
You can change your button from an „Email submit button” to a plain old „Button”. Then in the Object tab specify the Control Type as „Submit”. This will give you a Submit tab.
On the Submit tab set the Submit to URL field to be mailto:mailaddress (for example: mailto:me@mydomain.com). In the Submit As dropdown select PDF.
By using a regular button (over an Email button) you get the additional control of the Submit As.
2
You can still use the standard Submit Mail button, but you will need to make one small change. Select the Submit Mail button in the „Design View” tab. Then, click the „XML Source” tab. You should enter the source code with the „field” tag hilighted. Search below this tag for the „submit” tag. In this tag, you will see a parameter for „format”. The default is 'xml’. Change this to 'pdf’, and the document will be sent as a pdf doc.