It may have already been said, my apologies if so...
Tire manufacturers provide Load and Inflation information such as
this example from Firestone.
You must then inflate your tires (cold) based on the weight they are carrying (in a single or dual configuration). They all have a minimum pressure (as well as maximum). In my experience, the ride is always a bit less harsh at lower than maximum pressure (but at a pressure that is appropriate for the weight).