Feed the triangle wave and a reference voltage to a comparator (another opamp can be used for that). The comparator output is pulses with a duty cycle determined by the reference voltage (which can be adjusted).
Depends what your PID controller will be controlling.
A simplistic way would be to use a potentiometer for the P signal, an RC circuit (series resistor, shunt capacitor) for the I signal and a CR circuit (series capacitor, shunt resistor) for the D signal.
I suggest you read up about PID controllers.