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Jodofin - you must have Sumatra as the default PDF handler in your OS settings, if you change that to whatever (Acrobat, PDF XChange, ...) calibre will use it.
Or Calibre itself has a built-in OpenWith, i.e. no need for a plug-in. Right click on a PDF in Book Details and follow your nose.
Chances are that calibre will have detected all the available PDF readers and configured them. FX: I didn't add Chrome and IrfanView as PDF readers, they set themselves up as PDF handlers, calibre detected them, bingo I can use them from calibre - PDF XChange is my default PDF reader.
You can invoke an
Open XXX With program from within View->Specific format. And you can assign keyboard shortcuts to Open With settings in Preferences->Shortcuts - as I have for PDF XChange.
Hmmm: Firefox and Edge aren't in the PDF list? I think I know why not Edge, to use it in
Open PDF With I need to tell calibre about EdgeLaunch, but why not Firefox - a backwards Quantum leap Ψ³
Tip: when you can't find something in calibre right click in the likeliest of places.
BR