In establishing whether or not a crime has been committed, one fact that must be obtained is "Who is the victim?"
The crime of theft usually reads in the corresponding statute something like: "Intentionally, knowingly, or recklessly taking property with the the intent to keep it without the effective consent of the owner." In such a case we must ask "Who is the owner?" because that would be your victim.
It appears what was taken were files involving congregation disciplinary matters. It would therefor be reasonable to conclude that the owner of the files is the local congregation - a legal corporation. One would have to read the corporate by-laws filed with the State to see who is authorized by the congregation to be in possession of said files. Based on the nature of the files I would imagine there is no mention of such a thing in the corporate by-laws.
We know that any congregation member in good standing is has voting rights. That makes any member in good standing a part of the corporation. I was never an elder, but I was a Ministerial Servant. As such, I was given a key to the Kingdom Hall and the alarm code. I had to pick up literature from the Assembly Hall and deliver it to 3 different Kingdom Halls. So, you can see this all muddies the waters when it comes to who is authorized to possess such documents.
I didn't get the impression the person in question burglarized the Kingdom Hall. It sounds more like this person just walked in and took the documents. It doesn't sound like the local elders found the front door of the Kingdom Hall busted open and the file cabinet pried open as well. It sounds more like an elder was just going through files and noticed some missing.
Either way, on the surface, it does sound like this person committed Theft. The crime of Receiving Stolen Property usually reads in statute that the offender possesses property he knows is stolen or would be reasonable to believe is stolen (e.g. buying a TV from a guy in the back of a van in a dark ally at a ridiculously low price). Whatever the case, it does put the Watchtower in a bind.