Bill Thought I'd try Okular, https://okular.kde.org/ since you mentioned it, and since it runs on Windows as well as Linux (and I use Windows).
From my very limited trial of one document.
It feels a bit like SumatraPDF, just with a better interface and more capabilities. The rendering times are occasionally fast, but usually about as slow as SumatraPDF, with occasional very long white-out periods on zoom and always poor fill rendering quality. For use with Therion, if a viewer is a bit slow or poor at rendering that does not worry me much. My main reason for choosing a PDF viewer for use with Therion is that it allows Therion to update the pdf file while the viewer has it open. SumatraPDF achieves this on all but the largest of files I have Therion create. Okular fails in this respect with a modest file, with this error - indicates an application has the output file locked. C:\Program Files\Therion\therion.exe: error -- filesystem error: cannot remove: The process cannot access the file because it is being used by another process [Output/LaghuElevEXT.pdf] writing xtherion file ... done Okular has quite a few settings, so it is possible that changing some of those will remove this limitation. Anyone with insights on this? Bruce