Greetings!
A completed document in the Adobe Sign system cannot be edited by default due to a randomized password that is applied to the document as part of the certificate seal.
Enterprise level customer Admins can contact their Success managers and request that a static password be applied to all of the PDF copies that are generated on the account.
Once the static password is installed, any subsequent PDF copy pulled from the system will be sealed with that password, which can them be used to unlock the security of the PDF.
Worth noting, PDF copies are not the legal records. They are copies of the legal record stored on the Adobe Sign system. There is no way to edit the authoritative original.