When a user fills out a form on our site to join our mailing list, I’d like to send them a “thank you for joining the mailing list…“ e-mail with a PDF attached (the PDF being the incentive to join the list). Is there any way to do that from within Freeform?

To be extra-clear, this is NOT an attachment of something they uploaded. It would be an arbitrary file that already exists on the server.

I don’t believe there’s any way to do this “out-of-the-box” with Freeform… although it sounds like it shouldn’t be that hard to do with some custom coding.