Here's a link with instructions on how to enable Adobe Reader as the default PDF Reader, and disable each browsers default PDF "Viewers".
Acrobat Help | Display PDF in browser | Acrobat, Reader XI
IE doesn't have a built in PDF Viewer, and other browser's PDF Viewers such as Chrome and FF, don't allow submissions.
Note: Adobe Reader users need to have the form's usage rights extended so Adobe Reader users can submit PDF format. Otherwise, Adobe Reader users are limited to submitting data formats only (FDF, XML, XFDF, XDP), not the whole PDF.
Also: For compatibility and security reasons you should set the submit button URL to point to a server-side script such as PHP or ASP.net, and bypass client-side email software.
For examples on how to submit to a server-side script check out the following:
http://www.pdfemail.net/examples/