Before Cooper made his iconic jump, he left a few things and one of the things was his tie.
Smoking on planes was a thing in the 70's but even the fingerprints and saliva couldn't help find him.
More Evidence
Money with matching serial numbers from the ransom found on a small beach named "Tina Bar" in 1980. The rubber bands were intact indicating it was placed there recently by possibly the man himself.
Cooper requested 4 parachutes along with his ransom. He took 2 parachutes on his iconic jump and left 2 behind.
D.B. Cooper's Boarding Pass