Do you need an actual attachment or just a link to the PDF? It would seem that your requirement is an internal affair, which means you could just hyperlink to a file that you have already placed in a document library somewhere....
What I mean is that if a user attaches documents to your form you use SP web services to push them into a document library and only keep the link to them, then those links can be used in you email.
If your email is going external to your organisation then that can become tricky! But not so much that its impossible