If a door doesn't completely clear the surface of a newly laid carpet, you need to sort it out by trimming the bottom of the door. Opening and closing it will not only be difficult, but doing so will soon flatten the pile and wear an unsightly semicircle in the doorway.
On the surface
Place a strip of 3 mm thick hardboard or plywood on the surface of the carpet, butting against the closed door. Use the strip’s surface as a guide and scribe a cut line across the door with a fine-tipped felt pen. Do not flatten the pile by pressing down on the strip while doing so.
Open the door all the way. Support its weight by tapping a pair of wedges under the edge of the door. Ask a helper to steady the door and remove the hinge screws from the jamb with a cordless drill/driver and screwdriver bit.
Lay the door on a pair of saw horses. Use a steel tape to measure the distance from the cut line to the top of the door on both sides to ensure that they are the same. If they differ, erase the old cut line and draw a new one using the greater of the two measurements.
Place a straight edge on the cut line if it is a hollow core interior door. Use the straight edge as a guide and cut through the veneer with a sharp utility knife. This will prevent the circular saw blade from splintering the edge of the veneer during cutting. Skip this step if you’re working with a solid door.
Measure the distance from the inside edge of your circular saw blade to the far edge of the base plate. Transfer this measurement to the door above the line and clamp the straight edge on this line to act as a fence for the circular saw.
Put on a pair of safety glasses and turn on the saw. Trim a thin strip off the bottom of the door while sliding the edge of the baseplate against the straight edge.
Smooth down the bottom of the door with a sanding block and 120-grit sandpaper. Cover the exposed wood with coat of polyurethane varnish or enamel paint to match the finish of the door. Allow adequate drying time and refit the door by reversing the procedure carried out in Step 2.