Have you ever shortened a veneered door only to discover that when you cut it at the bottom the veneer splintered and you ruined the door? There is an easy way to avoid that.
Once you have decided how much to shorten the door, draw a straight line where the door needs to be cut. Then clam a straight edge along that line and using a utility knife cut through the veneer along the line. Then measure your circular saw from the edge of the guide to the blade. on my saw it is 1.5 inches. Then move your straight edge that distance and clamp it to the door once again. Cut along the straight edge and you will have a clean cut. But make sure that you cut is just on the outside of the line where you cut through the veneer.