I would say, this is not the best idea. Why not sending HTML instead? Less trouble, more flexible and adequate format, especially if you already have HTML, besides, HTML is directly rendered by all non-nonsense mail programs, PDF is not, and a PDF viewer may not be even installed.
However, if you still need it, you can use iText, or its .NET port, iTextSharp:
I provided the link to iText as well, as most documentation is for Java, but you can easily understand it.