I do not think you can scrap a PDC in the way you do for ships to gain back some of the mineral cost, you can use the ships screen (F6) to either destroy or delete the ship. I think delete simply removes the ship from the game, where destroy is like ordering the crew to scuttle it. This leaves you a wreck you can salvage for parts/minerals.
As for moving them you cannot use a tug etc. They can only be moved when in their prefab form via cargo bays, you then assemble them on site and they are now a permanent fixture.