By its very nature, the 555 timer produces a pulse at its output, either astable or one-shot, depending on component structure. The voltage and current supply at the output is a function of the voltage used to power the device.
PMW (pulse width modulation) is the varying of the percent of on and off times of each pulse cycle. It has nothing to do with the original output values. Therefore, PMW in itself will have no effect on the brightness of a LED connected via current-limiting resistor to the output.
A quick way to see PMW in operation is to set up the 555 to produce a blinking LED with a frequency of some four or five flashes per second.
Then, connect the ends of a potentiometer to V+ and ground. Connect the wiper to pin 5 of the 555. You now have voltage controlled PMW. As you vary the pot, the LED will blink slower or faster, depending on the voltage supplied through the pot.
When the LED blinks slowly, it is because the high portion of the squarewave pulse is very narrow with respect to the off portion of the cycle. Conversely, the LED will blink faster when the on portion is greater (wider) than the off time.
Hope this answers some of your questions.