@sigbyblue
You can download the Certificate of Completion (CoC) directly from the DocuSign platform, if you were included in the signature workflow.
It sounds like you have not been in the signature workflow. Therefore, you need to request it from the sender or another recipient in the workflow. If the sender refuses to provide it, you may get if from another recipient., but as you said it sounds suspicious and you need it get more details on who signed the document. DocuSign will not provide the CoC to you.
You can check the validity of the document and that is has not been tampered with. Open it in a PDF reader and review the embedded certificates. If the signature workflow leveraged digital certificates. They will also show up in the PDF reader and you are able to validate, which person has actually signed the document.
If they used a standard electronic signature, you will find some more details about the signers in the CoC, but no personal certificate will be embedded into the PDF. In this case expect only the DocuSign platform seal when reviewing the certificated in a PDF reader.