Copies are not acceptable. If the inspector wants to see your certificate, then he wants to see the certificate, not a copy of the certificate. The certificate is not yours, you are in possesion, but it actually belongs to the US government. In other words, if you don't let him examine the original certificate, then you are in violation. The regs don't mention that a copy is acceptable. Same thing goes for the airworthiness certicficate or registration certificate in the aircraft.