Hi Amal,
So actually, I still can add a signature with this tool:

on a document than has been signed with this tool ?

Thank you & regards
Noko
Hi Noko,
Hope you are doing well. If the PDF is sent to you via Adobe Sign with a signature field assigned to you, then yes you can add the signature to the document which is signed (Digital/certificate signature) by first person.
If the PDF is sent to you an an email attachment (No signature field is assigned to you), then you can not add the signature to it. As once the PDF file is signed and the signature process is completed, it gets locked for further editing and we can not make any more changes to it.
Hope it will help
Regards
Amal