The HVAC system is fairly good about trying to blow air into a room IF the ducts are sized properly, AND there's a way for that air to escape...it takes the easiest path out of the ducts, which may be a room below. Depending on how much room there is under the door, you migth get some flow under there, but it is unlikely the proper size. There are several alternatives...add an air duct either in the wall, or the door. If the doors are never closed, it's not a problem.
The other issue is, especially when running the A/C, that heat rises. If you don't get any flow, the room will be uncomfortably hot in the summer. I've found my system is more efficient if I have a return duct drawing air from the high point in the house. Rather than drawing cold air out of the bottom, it draws off the very hot air at the top, whether from the hot air during heating season accumulating at the top of the house or from heat rising during the cooling season...drawing that hot air off from the top (and running the fan most of the time; i.e., not on auto), evens out the air temperature. As to humidity, it is normally the same around the house except when using something like the shower or dryer. But, different temperatures will reflect different relative humidity levels, and a cool room might feel clammy verses a warmer room with the same actual humidity in the air (note the difference between relative and actual humidity in the air). Warmer air can hold more water, so the same amount will represent a lower relative humidity level.