Ok yes the doors can
be adjusted but you are best letting a body shop do it. Before I discuss some of
the methods be aware it is possible to get the adjustment so far out that the
door locks shut, so before you start remove the interior door panels.
The bolts holding the hinges on many models are made of pot metal or some alloy. As a result they really are a one time tightening and never to come loose again. If you take an impact wrench to them you will just make a mess. The only way I have found to get them loose is to used some heat - well actually quite a bit of heat such that the paint is burnt off the hinge mounts and door jamb. You can try asking Jim at 928 International if they have come up with a better method but I have not found one.
When the hinges are loose there is some play allowing the door to be tilted top to bottom. Often the door if remounted will be tilted away from the top door frame and wind at high speeds will make a noise. I have found using some thin plastic rod that the top of the door gasket can be adjusted out to make a better and tighter fit.
Adjusting the door is best done first with the door gasket off and then when the lines are right with the gasket in place. The height of the door in the frame can be adjusted by raising the hinges higher in the sockets. There is a tool made for holding doors in place but I use a floor jack and a couple of people. Like installing a door in a house - something some people make a career out of - adjusting a car door is more art than science.
Now that solves the tilt from top to bottom but you can also adjust the arc by tilting the hinges from top or bottom. All these adjustments are minor and assume that at one time your Porsche was not hit by a train. Over 80% of the 928s on the road have been in at least one accident. I once figured the mean mileage between accidents for a 928 as about 20,000 miles. So if your car has some mileage and the doors are not fitting properly then it is time to measure the frame - like on a body rack.
Now for the hacks - which I am sure is what you are after.
If the door is too far rearward and hitting the jamb before the latch it can be pushed forward with a porta power. The trick on the 928 is finding a leverage point that will not damage the body further and be strong enough to move the door post forward. Pushing at the hinge will move either the top or lower hinge forward creating more clearance as well as tilting the door up or down. I will leave it to you to find a porta power and place to leverage it. But then if you had the skill to try this trick I suspect you would not be asking the question in the first place. :) Kids do not try this at home.
Ok if the door springs a bit when closing then it is too tight at the hinges at the door post. The only body man's trick to fix this is to put a 2 x 4 between the jamb and hinges and close the door. But before you go and stretch the hinges make sure there is not some other obstruction in the door jamb. This stretches the hinges and causes the door to move rearward and close closer to the rear jamb. Again kids do not try this at home. It works but you need to be careful and it takes a bit of practice.
Finally there is the possibility your hinge pins are shot. This means the pivot on the hinges is worn. If this is the case - well you are F&*(ked as the hinge is part of the door. You can not take hinge pin sag out just by jacking up the door. You might be able to take the door off and drive out the hinge pins and put in new ones. But the hinges are also probably worn too. Since half the hinge is part of the door I do not hold out much hope but for a new door.
Normally 928 doors are easy to adjust. If yours are so far out as to not respond to a little tweak here and there then your car has some serious former body damage or ... well you need to see a professional.
Hope this is helpful
Dan The sometimes pod guy