I used to work in that area. I'm not familiar with that specific address, but it's in the same general area (within walking distance) of Nissan USA's national headquarters now that they've fled California, and the Ford Motor/FMCC office I worked at. It's a mixed-use area with lots of shopping, restaurants, office buildings, import car dealers, and a handful of expensive condos -- your typical suburban sprawl. It's not your average 'industrial office park' area with low rent either... that's not a cheap zip code. Just a guess, but it is probably an office building with 24/7 security who may do some long-term document storage for clients, unlike some of the more specific document storage specialty places like Richards' that is downtown who may not have around-the-clock guards.
It might be a possibility that Romney's tax returns wound up there but I don't know enough about PCW to comment. I have no idea what PCW actually does, so the entire thing could be a fabrication.