I look at this way...all the guys in the files? I'm betting if you looked at all the jump schools in the surrounding states you would find hundreds more that could be suspects, probably more..then lets take a look at the military files...how many jumpers do we have there that fit the description or skill sets? then look into all the aviation angles..it's possible Cooper was none of the above?
Cooper's description is not an uncommon one nor is the sketch. a scar or a crooked tooth could of sure helped out in this case..
we can read file after file of people calling or writing the FBI with someone who matches the description or sketch. we have a rather large pool of Coopers...an ocean of them.
Is it possible he is in the files, sure, just as possible as he died or lived....
My view is quite different:
1) Cooper knew about the 727 somehow. This limits a significant number of people.
2) Cooper was familiar with the Seattle area. This limits a lot of people.
2) Cooper likely had jumping experience. This limits a lot of people.
3) Cooper had to be in a position to acquire the tie particles. This is very limiting.
4) Cooper had to have an alibi. If Uncle Bob was in Memphis for Thanksgiving, he's not Cooper. This limits a significant number of people.
5) Cooper had to have a certain personality. This is quite limiting too. Most people would be viewed as "no way in hell Jim would do something like that."
6) Finally, Cooper had to roughly fit the description...mid 40s, 6', in good shape, black hair, high hairline, American or someone who spoke with no accent. This is actually very limiting too.
When you apply all of these criteria how many suspects can there really be? Truth be told, not many.
Then, of course, there are going to be other things such as DNA, unexplained money, etc.