I'm not sure you mean "what files to download" as there really isn't anything to download directly. Some guidance can be found here (
Loading Firmware onto the flight controller — Copter documentation). You do this with the Solo powered off, and just the pixhawk plugged in by USB. Just follow the prompts. If it asks you if this is linux board, the answer is no.
Since I don't know what state it is in, I suggest installing ArduPlane. That will wipe out the pixhawk completely of whatever was messed up on it before. Once that is loaded, unplug it from the USB and power the Solo up. Try pairing and connecting. Then run through the whole update process through the 3DR app.