Each envelope has a History and Certificate of Completion associated with it, this is found under the envelope if you are a Recipient or Sender within that envelope. Locate the envelope under Manage, Inbox or Sent then go to the dropdown to the right of the envelope and select History. When the History opens you can select to download the Certificate.

Here is a DocuSign article on the envelope History:

https://support.docusign.com/en/guides/ndse-user-guide-history-coc