Landing Forces and Paratroopers act like ordinance such as torpedos and shells.
Each object delivered to the correct area deals damage, once dropped, they are impossible to stop (Except maybe if you sink the APA or CV, Its been awhile since I've seen one killed while dropping paratroopers or landing forces. It may act like planes and disappear when they die, or they may continue.)
The damage dealt per individual object (Individual Paratrooper, Marine/Soldier, or tank) is based on the true ability of the sailor the object belongs to.
Just like shells can't hit other shells in mid air, shells can't damage paratroopers or landing forces.