A back back, that can be attached to the seat post: "KLICKfix Rixen&Kaul" either Meta or city. They have a sheet of fiberglass in the back and I find this is good to prevent papers getting bent, although there are two nuts that stick inwards in the bag. I use one and carry, just checking: a set of tools, oil bottle, oily rag, rain jacket, first-aid kit, iPod, homewards set of riding kit, a spare head lamp for if the main fails, a spare water bottle. Normally also carries a cable lock, but that's locking up the bike currently, and some papers which're with my secretary and I'm surfing here...

If you need to keep the papers pristine: another thing I use is a "Fast Rider" pannier, this is like a brief case that has a flap to hide the handle/hide the panier hooks. It is very stiff, which is better for keeping papers straight, and for carrying more papers or a laptop, but can't really carry anything like a sandwich box as it'd pop the lid off.