Not all. There should be resources showing the generalized power factor too. Reactive power is the linear / AC steady state case, but it works for any components orthogonal to the source (which, cosine is orthogonal to sine, so reactive power is an example of this).

That's probably not a great approach, as you need the Fourier series and Parseval's theorem to find the orthogonal (harmonic sine waves) components, and then to add them up.

If nothing else, you can take S = Vrms * Irms (input values), and P being the output power (assuming no losses elsewhere -- no voltage drop on thyristors -- if this is consistent with the material?), then PF = P/S.

Which, because Irms is the same (input and output) in this circuit, and the load is resistive so Irms = Vrms(out) / R, it's simply Vrms(in) / Vrms(out).

It's maybe not obvious that the second (ratio of RMSs) is equivalent to the first (doing component-wise analysis; though Parseval's theorem basically being RMS over the spectrum, which might be a pretty big hint), so a proof would be instructive.

Tim