Stands for PortleanD eXtended erea. I don't know if similar tricks are possible in other areas, but 'round these here parts, if you know an adress to be in this area, but don't know the exact town, or zip, 1 can often send mail to:(adress) pdx, 97208, w/ out much of an ordeal. In fact I think it often works w/out an adress.
This reminds me to check into what a postmaster general does. I've heard there are several situations in which they can be quite useful.