A space is pressurized by exhausting less air than is directly made up. The 'extra' air pressurizes the building as it escapes through the cracks and penetrations of the space.
In your scenario, if 500 cfm of your fan was supplied from outdoor air, and only 300 cfm was exhausted you would have a general pressurization of the space. How much pressure would depend on the size of the space (specifically the leak paths, building orientation, wind speed and direction and a half-dozen or so other variable).