How to reduce the amplitude of my PWM signal?
Not the voltage regulator. That will try to give a constant voltage at the output, but PWM at the input won't allow this. Anyway, the regulator won't work, but your PWM signal will be lost.

A potentiometer will work, and allow you to vary the output level, but if you know what level you want (5V) then a resistor divider is cheaper. 3k9 for the higher resistor, and 1k for the lower one will give you 4.9V. Both values are E12, so they are more common that the values Nick suggests.

edit
Nick suggests higher resistance values, like 39k and 10k. Lower values mean less susceptibility to noise and other disturbances, and if you're working with stepper motors you can afford the extra few millis; the stepper motor will consume a lot more.