From FAAO 8900.1:
5-196 PERSONAL POSSESSION OF PILOT CERTIFICATES. Title 14 of the Code of Federal Regulations (14 CFR) part 61, § 61.3 requires that pilots of U.S. civil aircraft have a current pilot certificate in their personal possession when acting as a required pilot flight crewmember. This means that, to avoid the possibility of fraudulent use of another’s pilot certificate, the pilot must be in possession of an original certificate, not a copy. The copy of a temporary certificate issued to an airman by the FAA or by a designated pilot examiner is acceptable; however, inspectors should check all temporary certificates for expiration dates.
NOTE: Airmen may either use clear laminating sheets to protect permanent FAA‑issued certificates or have the certificates professionally laminated as long as the airman’s signature is placed on the certificate before lamination. Without the signature, the certificate is not valid.