Well, first of all, it was necessary for "Gator" to discover them by accident, so for the plot this was the only reasonable place for his files to be. But the obvious plot-device reason out of the way, we might still be able to come up with some possible motivation for Broker to keep them there (even if maybe with a bit of a speculative by-taste).
As you mentioned, Broker had finished with his life as undercover agent and wanted to start completely anew. Thus he also didn't want those sensitive files to lie around in some government office accessible to anyone with the right permissions. Part of his complete departure and disappearance from the DEA and his previous life was that those files and thus Broker's previous identity vanish from existence. But then again, he might not have wanted those files destroyed completely, be it out of sentimentality or a fallback hold to his previous life, or just pride in his achievements as a DEA agent.
So the only alternative to destruction was to take care of those files himself, so what about a safe deposit or locker somewhere? But Broker is certainly someone to take things into his own hands and he explicitly took this new home far away from civilization because it was a remote and safe place for his family, so why not for those files either. The inviolability of homes was one of the few things Broker still could and had to rely on. There wasn't anyone looking for him there anyway and if they were, they wouldn't have needed those files anymore.
What he simply did not expect was someone invading his home, accidentally stumbling across those files, and knowing what to make out of them. You might call this an error on his behalf, be it out of carelessness, too naive trust in his new start, hubris in his abilities to care for things on his own, pride in his previous life, or simply a talent to get himself into trouble again, as he did with gaining too much attention from the locals in the first lace. But it certainly happened and it had to happen as a wake-up call for Broker to finally finish with his previous life and for the real clean-slate to start his new life.