It depends on what type of button you are using. If you are using the "submit by email" button, in the Object tab, directly under the "email address" and "email subject" fields there is a "submit as" drop down field that prepopulates as XML. Just select the drop down and select "PDF". If you are using a standard button, you first need to select "submit" in the Object tab, then in the Submit tab again select in the "submit as" field PDF.