For regulatory reasons, we are using token-based digital signatures with embedded (time server) timestamps to sign PDF files.

When using Fill & Sign to request external signatures, we need to be able to:
(1) digitally sign the PDF document (using our tokens) before it gets sent out, without the signature being discarded by the Adobe Sign service, and
(2) digitally sign the PDF document (using our tokens) again, after all signatures have been collected

I was very surprised that this was not possible.