I recall reading an account of this incident and Cooper apparently instructed the pilot to slow to minimum controllable airspeed just prior to his exiting the aircraft. Exiting the aircraft at that speed, roughly 130-140 knots, wouldn't have been a problem. This is the problem:
Cook said his research has shown winds were blowing from the southeast at more than 60 knots in the Portland area around the time of the hijacking.
Trying to land a parachute in 60 knot winds is suicidal. The chances of surviving a parachute landing in this kind of wind and not getting seriously hurt of killed is slim to none......
The FBI rigged the chutes. IIRC, one was rigged not to open, the other had it's cords cut. The other two, which worked, were found on the plane.
My guess is that Dan Cooper attached a flare to the money bag, and was planning to use the steerable parachute to follow it down.
I guess the phrase, "don't judge a parachute by it's cover" would have been helpful to Mr. Cooper.
According to the details given by the FBI, it would appear that Dan Cooper, or D.B. Cooper, dropped to the ground like a rock. That's probably why they never found him. He torpedoed right into the ground like a lawn dart dropped from thousands of feet.