The mailto will not change the fact that you cannot submit the PDF ...it will also only submit XML. You will have to Reader Extend the form to allow a local save before you can submit a PDF.
Back to the mailto .....do you have a mail client on your machine (liek Outlook or Thunderbird) or do you access your mail through a web address (like gmail or yahoo)?
For this particular project I am using Outlook. I also use Thunderbird though, and web mail.
What do I do to Reader Extend the form?
If you have Acrobat you can open the form in Acrobat and under the Advanced menu selct the Extend Features in Adobe Reader option. Just follow the wizard, then use the resulting PDF that is created.
The other option is to get a server product called LiveCycle Reader Extensions but this woudl be overkill for what you have described.
If you do not have Acrobat, I suggest you get a trial version from the Adobe web site and validate that it will fix your problem. If it does then buy one copy so you can Reader Extend your forms.
Hope that helps