The screws that hold the pneumatic opener/closer to the screen door are loosing up and soon will fail. The holes in the aluminum door are stressed and enlarging. How can I fix? Should I drill thru the door and attach with a longer screw and nut?
The best bet with this is to acquire a small sheet of plain aluminum. Consider material with a thickness of say 3/32 inch. Cut this to a strip that is about five times longer than the existing closer bracket mounting area. Make it as wide as the door web to which the closer mounts. Drill clearance holes in this plate at the ends (corners) and then drill proper thread tapping holes into the door web and mount this sheet centered over the existing bracket mount location using quality sheet metal screws into the door.
Finally you can mount the closer bracket into this new plate using some new sheet metal screws. The holes in the new plate have to be sized to allow the new screws to cut threads into the aluminum plate.
I'd simply shift the mounting bracket a half inch in the most appropriate direction and mount it with new, properly-piloted holes and fresh screws. In most cases these brackets have a long adjustment slot which should facilitate this just fine.
Otherwise, reinforce the area as Michael Karas suggests.