Droopy doors that rub on the floor can sometimes be fixed just by tightening the screws on the top hinge. If that doesn't take care of it, Ask The Builder suggests installing hidden screws.
Door hinges become loose through normal everyday use or poor workmanship. Tightening the screws is the simplest solution, but you can also install 3-inch hidden screws under the top hinge, which will prevent future problems.
These simple steps will solve almost all drooping door issues. The long screws under the top hinge will almost always prevent future problems because they ensure the door jamb will not succumb to gravity.
Hopefully tightening the hinge screws will take care of your droopy door. If it doesn't, hit the source link for more details on installing the long screws.
How to Fix a Drooping Door [Ask The Builder]