"...And even if he has some documents that he shouldn't, is it a...

"...And even if he has some documents that he shouldn't, is it a criminal act that can be proven in a court of law?..."

It doesn't matter how many documents, it was still illegal for Trump to take any of them.

Yes it is a criminal act.

Up to the powers that be to ascertain if it warrants prosecution in a court of law.

Whether they do or don't prosecute Trump, it does not change the fact that Trump took documents that he wasn't legally entitled to take.