Why is there a capacitor and resistor added to this voltage quadrupler?
The capacitor is there to provide additional ripple smoothing.

The resistor is there to discharge the capacitor when the circuit is powered down.

They are commonly referred to as "Bleed Resistors".

Usually they are for user safety. The capacitor could retain enough charge to cause an electric shock for a long time after the circuit is powered down. The resistor is selected to discharge the capacitor to a safe voltage after a few seconds.

The value of the resistor in the diagram (82K) seems rather low considering the voltages and would dissipate a lot of power. What is the value of the capacitor?