Pneumocystis is not a fungi, but does stain with GMS,  

Periodic acid may work for some fungi, but is a weak oxidizing agent, compared to Chromic acid.

I never had good success with it, and some fungi might not stain as well after using the PA, in
lieu of chromic acid.

Even when using Schiff's reagent for strong staining of fung, the chromic acid is used as an 
oxidizer.

Caveat here, calling this combination a PAS is not true, it is a chromic acid-Schiffs.

I think there was discussion about this a year ago, or so on HIstonet.  and there may be a
publication lurking around, as I recall???? in Journal of Histotechnology.  

Beware of creating a false negative because of using of weaker periodic acid oxidizer, 
you might miss something -  a fungi wall better oxidized by chromic acid.  

Gayle Callis