Undervolting PWM fan from 12V to 5V
Running a PWM controllable fan designed for operation at +12V may run OK at a lower voltage as long as you do not go too low. PWM controlled fans have an IC chip inside and one or more hall sensors. When the applied voltage to the fan gets too low the IC chip will fail to operate correctly. I have found in some experiments that I have done that I was able to get the fans to operate OK at voltages down to about 6.2 volts. Below that the fans would not start reliably and the fan motor lacks torque. 

I would personally recommend that you operate a 12V fan at 12V and then use the PWM feature to regulate the fan speed to the level you require.