This works if the oscillator in the PLL has much less jitter than another source. You lock the PLL with a very narrow loop filter.
One example of this is in crystal oscillators. Highly stable crystal cuts have more phase noise than less stable cuts. So you set the frequency with the stable cut oscillator and then follow it by the PLL with the low phase noise crystal cut and use an extremely narrow loop bandwidth.
One way to reduce the jitter in digital data is to use a D-type flip flop. Output jitter in this case is defined by jitter of the clock of the flip flop plus inherent jitter of the flip flop.