But you don't get it, it's not the problem of buying a resistor or not... I was just wondering "If PWM output is set to 66%, as if it was outputting 3.3v, why add a resistor, since it will drop voltage even more so the LEDs won't be at full brightness?"... But now (after re-reading entirely this topic) I think I understood 
I will do what dhenry said in his last post, and use PWM's max value instead of trying to limit it.
Thank you