A portable circular saw is the tool to use when you need to shorten a door to fit an existing opening or to accommodate a new layer of thick carpeting. But, even the newest, sharpest blade will tend to splinter the top surface, especially if it's a veneered door.
To eliminate the splintering, first apply a strip of 2-inch-wide masking tape across the door. Clamp a framing square or other metal straightedge along the cut line. Then, slice through the tape and into the door using a utility knife. You should make several firm passes.
Remove the straightedge, but leave the tape in place. Then clamp a perfectly straight 1x4 or 1x6 to the door to act as a saw guide. Position the board so the blade will cut about 1/32 inch to the waste side of the scribed line. Slowly guide the saw along the board to trim down the door. Finally, pull up the tape and lightly ease over the edge with a sanding block.
This no-splinter sawing technique also works well when crosscutting sheets of plywood and wall paneling.