3DR PIXHAWK HOW TO DO this

Can I use a pwm receiver?

Not directly.
There is an add on board that 3DR were selling that is a PWM to PPM convertor.
You will need something like this as the Pixhawk is PPM in only.

Such as this one from Usmile. You can probably find this even cheaper elsewhere.

I looked in to one and ordered it.
Thank you
Terry