MMS doesn't actually "attach" files. The protocol requires that the file reside somewhere (usually on the sending phone's provider's network), and contains the URI where the recipient device can find that content. It's then fetched over the Internet by the recipient's phone and embedded into the message. That's why it requires a data connection.
I don't believe that PDF is one of the supported MIME types for MMS, but I could be wrong. My recollection is that it's not.
If the documents are non-sensitive, you could probably store them on your server with a long, randomly-created, hashed file name and URI, and just send the link by text. There are many services that allow you to send text messages from a PC. The PDF wouldn't render in the MMS interface anyway if the MIME type isn't supported by the protocol.
Rich