is there a PDSF manual for RT 5.9
There’s no up-to-date PDF for version 5.9.
RawPedia has a link to a PDF, but that one is from November 25th 2020 (this one). It does deal with RawTherapee’s basic/general principals, though.
Not sure if you are aware that there is a on-line manual for RawTherapee (multiple languages) that is reasonably up-to-date. English version to be found here: RawPedia