My boss received a letter addressed to him, it was open and laid out in plain site for anyone to read. When he asked me put a file folder on his desk, it pertained to me and my personnel file which was false. Was it wrong for me to read this or just not morally right?

Don't confuse wrong with illegal. While illegal stuff is wrong... not all wrong stuff is illegal.

Tread carefully however because most places you are an AT-WILL employee... even with a Union... and you don't even have to do anything wrong to be terminated.

As smoothy said, it wasn't illegal, but it was snooping.

I'm not clear what the letter was a about though. You said; "When he asked me put a file folder on his desk, it pertained to me and my personnel file which was false." That sentence makes no sense.

You violated his and the companies right to privacy. This could be a tort (or civil) crime which you can be sued for. It is very likely a violation of company policy and trust which you could easily be fired for.

As for as criminal, to be arrested by the police, most likely no

You say it was addressed to him. As soon as you saw that at the top, you should have stopped.
Illegal, immoral, unethical? ... just plain wrong.
If you are wondering if it's OK to bring it up given that it's about you, the answer is no. Especially given that it might have been planted out 'in plain sight' deliberately to see if you would read it and protest.