Both Periodic Steady State Analysis (PSS) and Periodic AC (PAC) Analysis, comes under SpectreRF Tools, which is equivalent to transient and ac analysis respectively in low frequency application.
the theory behind them is the iteration theory from maths, e.g. the newton iteration method.
generally, we run the small signal analysis on top of the stabilised dc analysis. thats why we run the dcop first, followed by small signal analysis.
now bout the pss. it is still a dcop. but, it changes with time. so pss analysis tries to do dcop at every point, then pac will follow it by doing the ac analysis on top of the iterated point.