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mitch13 - so what does it do mate - chapter and verse
If you press 'O' to open book folder, and then double-click on the PDF what happens. I'm not Windows 8.1 au fait, but if it does not open it up in Acrobat then its the very likely Default Programs settings in Windows that are the problem. Fix that then its likely that opening PDF's in Acrobat from calibre should also be fixed.
BR