Air coolers are OK to a point, but because they evaporate water into the air, they add to the humidity. This can make the room more uncomfortable than heat alone. Particularly on a day that is humid to start with.
Air conditioning on the other hand condenses any moisture contained in the air as it passes over the cold evaporator and removes it as water, thus improving the environment even on a humid day. Even the portable units do this. They have a water collection container that needs to be emptied from time to time.
Maybe a ceiling fan (or two) would achieve what you require. On a slow speed, they create an even, more gentle breeze than an oscillating fan and would be less likely to blow your paperwork around the office.