Options for carrying a smart briefcase on a bike (pannier or otherwise)
Depending on the thickness of your briefcase it may be a solution to hang it under the frame in the main triangle (if it fits there) but since you are thinking about a folding bike it is not a solution.

I've seen something else couple of times - a special carrier for briefcases:

 

I'm not sure how a folding bike would accommodate such a carrier as it attaches to the seat-stay and the back of the normal rear rack.



Another idea would be to put the front baggage rack and tie the bag there. Not only is this more stable (the bag is "in your hands") but you also have the bag (probably containing important documents) always in sight.

This is what I mean:

But again - folding bike limits your possibilities.