You dont need a PPM encoder for APM as it accepts PWM inputs. Bit if your receiver outputs PPM i would suggest using it as its much easier to connect as APM also accepts PPM if you short inputs 2 and 3.
Its the more recent flight controllers like Pixhawk that need a PPM encoder (if your receiver is only PWM) as Pixhawk does not accept PWM inputs directly, only SBUS or PPM.