PLS does not have preclearance. You must go thru immigration in Atlanta. BTW, on the way back there is a large upstairs airside section at PLS with a outdoor deck with a outside bar with a great view of take offs and landings. Enjoy the trip, Turks and Caicos is amazing. One tip, on the way down, buy a bottle of your favorite booze at the duty free shop. You can even buy online and they'll bring it to the plane. Prices are crazy down there!