Appears you may be either asking on 1 of 2 levels
EITHER
(1) You want to know about how power spectral densities are processed by Pro/Mechanica. If so then can't really help.
OR
(2) You want to know about 'power spectral densities' as in the principles/theory of. If you are asking about fundamentals such as this then this would indicate that you may be a novice in vibration analysis. Be aware that vibration analyses (at least in my view anyhow) can be very difficult to get your head round - professional engineer level understanding of Fourier transforms ...etc will be prerequisite to understanding what you are doing. It can all go horribly wrong without a measure of experience and significant depth of understanding.
There are many text books on vibration analyses. From my own POV I found the website