There are no real variations of the PDW. The PDW is a variation. Of the basic MP5. It's an MP5K with a side-folding stock.
It was put forth for REMF and SpecWar guys.
It would be best to use it without optics or lasers. The only mod I would recomend would be a slot-cut compensator. The weapon is meant to be carried in it's little quick-draw rig. Accesories would muddle the draw.