I just bought a used car. The owner tells me to Notarize the Penssylvania title? what does this mean?

Im a first time car buyer. I just bought a used car from someone. He was nice enough to leave the plates so that I wouldnt get in trouble but insisted that i Return them tomorrow. He signed the back of the title with the mileage and game me the title. He is now telling me that I need to notarize the title because they are from pennsylvania otherwise DMV is going to give me trouble because is not notarized. What does this mean? is there a way I could be scammed here? I paid him the money allready. Help, thanks.