Going by the datasheet, the fan isn't specified to work below 36V because it won't meet the specifications or the driver electronics inside the motor won't start reliably at lower voltages.
The fan contains a brushless motor with an electronic speed controller and the datasheet doesn't say anything about PWM control, so it might not work properly.
The datasheet mentions an option with a speed signal so your best bet is to use that, rather than PWMing the supply voltage.