A basic trend for acid: if the conjugate base is more stable, then the acid has a lower $\ce{pKa}$. Therefore, it would quite intuitive to think that resonance in the conjugate base will make the acid more acidic.
e.g. Cyclohexanol $\mathrm p K_\mathrm a$ -16 ; phenol $\mathrm p K_\mathrm a$ -10
But, there are exceptions. For $\ce{H3PO4, H3PO3, H2PO2}$ the effect is reversed. There is a more spacious resonance in $\ce{H3PO4}$ than in $\ce{H3PO3}$. But, still $\mathrm p K_\mathrm a$ of the former acid is greater than the latter.
Edit: Mithoron commented that my claim about P acids isn't true. Though I don't what claim he is referring, I think even his linked question supports my claim of the order of $\mathrm p K_\mathrm a$ as $\ce{H3PO4>H3PO3>H3PO2}$
So, my question is : can the effect of resonance on acidity be generalised?
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