The first thing you should do is check the door hinges to see if they are seated properly and also whether the screws are tight to the door jamb. Look closely at the tolerance, or difference, between the door and the jamb it closes against. Follow the outline of the door all the way up and down the door to be certain that the door and the frame are parallel and that any gaps are uniform all the way around the door.
If the two parts, the frame and the door, are not parallel, the door may need to be adjusted by adjusting the hinges. Loosening above and tightening below, as an example, may help the door fit better, but finding the proper fit is a matter of doing a little bit at a time and using small shims. I suggest that this job be done by someone who is mechanically inclined.
If the frame and the door are parallel, the door can be shaved with a sharp wood plane. Do not use a chisel, as there's a good chance it will gouge the edge of the door. Some craftsmen or women use markings in the area they wish to plane. This is done by determining the area of the door edge that is catching, rubbing the door edge with chalk or pencil lead or something similar, and planing away nothing more than the rubbed area. Do only enough to eliminate the catching, and feather out the planed area so that the door looks straight.
I recommend doing this procedure while the door is on the hinges so that you can quickly and easily check your progress to avoid overdoing it. Taking the doors off may be better for keeping the door in place on a sawhorse or a work bench, but it can also be a pain in the neck to carry the door back and forth, which increases the chances that you'll cut away more than necessary.
Thanks very much for your inquiry, and good luck with your doors!