I've always used a simple NPN transistor to buffer a logic signal to drive an LED.
If you can invert the PWM signal then drive the base through a resistor, ground the emitter and tie the collector to the cathode of the LED.
If you can't invert the PWM signal, use the NPN as an emitter follower. Connect the collector to 3.3v, drive the base directly, and connect the emitter to you LED resistor. The LED resistor may need to be a lower value as the emitter follower will reduce the voltage by about 0.7Volts.