PDF attachments

I'm trying to attach a PDF to a soap request.  When I receive the bytes in my service they seem to be corrupt.  Closer inspection of the generated soapui request shows that unicode replacement characters (EFBFBD) are being inserted into the byte stream.  The mime part headers for the PDF look fine:

-----=_Part_31_2003127473.1490924132210
Content-Type: application/pdf; name=my.pdf
Content-Transfer-Encoding: binary
Content-ID: <pdf>
Content-Disposition: attachment; name="my.pdf"; filename="my.pdf"

But the original bytes from the PDF and the bytes being sent by soapui don't match at all.  Can anyone shed some light on this for me?  Is this just not possible?  I could base64 encode the PDF and send it as text but I'd rather not.