DB Cooper has been the topic of Drop Zone conversations for years. The jump he made is perfectly doable by anyone trained in the practical application of aerodynamic decelerators. Albeit it was an extremely risky jump as I'll detail below.

As a matter of fact I've gone out the air stairs of a 727. It was at the 1999 at the World Freefall Convention in Quincy, IL. It was fun, but a 727 on jump run is doing about 180kias when normal skydiving operations are around 90kias. It won't injure you, but it feels like you're getting hit by a ton of bricks. So not the funnest jump and I only did one, to say I did it. Going out the bomb bay of a B-17 (Nine-O-Nines RIP) was a lot more fun.

The general consensus in the skydiving community is that Cooper lived, but was probably injured. He jumped a round parachute, at night, in poor weather conditions. Even with modern gear that kind jump is extremely risky. Round parachutes go down where the wind blows them after exit. There is a little bit of control with a round, but nothing like a ram air chute.

He probably had a vehicle stashed close to the drop zone and was able to get to it with most of the money. If I recall correctly some of the money was discovered on a creek or riverbank some time later. He probably lived the rest of his life in secure obscurity... Or he might even still be alive.

Only he knows for sure.