Are you saying a PWM signal comprises of all frequencies up to 490Hz (see attached)
Right idea, wrong on the details.
One of the central mathematical principles behind circuit analysis is that any periodic function (read: waveform) can be represented by a sum of sine waves of various frequencies, amplitudes, and phases. If you are able to decompose a complex waveform into its individual frequency components, it's easy to calculate the effect a filter will have on the waveform since you apply the filter to each frequency component individually, then add them back up to get the result waveform.
So it's not "all frequencies up to 490 Hz". Most of the frequencies will actually be harmonics well above 490 Hz. You have the right idea though.
and the filter 'highlights' the frequency we're interested in?
Not in the way you're expecting. The point of the low pass filter is to remove all off the frequency components as much as possible, leaving only the DC component behind.
But there's a tradeoff to this. Lowering the filter's critical frequency will reduce (but never eliminate) the effect of the higher frequency components, but it also means that the DC value will take longer to reach the intended level after it changes.
Depending on what you want to do with this signal, it might be better to get a DAC chip.