Modern atx power supply fans are exhaust fans. They pull warm air from the interior of the case and exhaust the air out the rear panel.
There is an exception. Some modern pc cases with bottom mounted power supplies have an opening in the bottom panel directly below the power supply. An individual has the option of installing the power supply so that the psu fan is directly over the opening. The power supply can pull in cool air from the bottom of the case and exhaust it out the rear. This will work if there is sufficient ventilation, air flow, and cooling for other components inside the case.