"To seal a document" means to close it securely using a stamp, signature, or other official mark to acknowledge its authenticity and prevent any tampering. Full definition
I directed my staff to go over to the office which holds sealed documents. (slaw.ca)
Other responsibilities of a paper & prints restorer include sealing documents in cellulose cases, restoring paper objects to original appearance or repairing objects by mending tears with adhesive and tissue. (topsampleresume.com)
They chase after a packet of sealed documents that could contain damning evidence. (combustiblecelluloid.com)