The PMOS is turned off when gate voltage reaches the source voltage.
If the source of the PMOS is connected to 8.4V, the gate also be 8.4V to turn off.
This means you can't connect gate to directly a 3.3V Arduino.
In that case, an NPN transistor (or NMOS) is used to configure a level shift circuit.
In other words, if the voltage you want to control with the PMOS exceeds the Arduino power supply voltage, you need a level shift.
(It is assumed to be an enhancement type MOSFET.)