Did they ever disclose how Arthur

No, and this has always bothered me.

My guess is that the "warehouse fire" Karen referred to was a deliberate attempt to destroy records. I'm sure Arthur had been told about that and did his own digging to find out where other copies of the records might be. The best litigators build their opponents case and their own at the same time, and those records would have been extremely damaging if copies came to light.

Arthur found them and it let him to unravel his entire life's purpose -- this wasn't just a law firm damping down a client's minor problems, but it was a law firm defending deliberate, financially-motivated toxic pollution.