PDF Document templates get attached to Emails as HTML file,

I'm trying to get a PDF Document Template to attach to an Email Template that is sent via a workflow action. I put the token {!registration_invoice} of the document template field into the email body. However, even though the document is set up as a PDF, it is being attached as an HTML document. This results in a unrecognizable data when the recipient goes to view the email attachement. Thanks