Adobe Acrobat opening PDF's

 On my PC, suddenly Adobe Acrobat has started opening PDF's.

 

 

 "Always open PDFs in Acrobat Reader" - I set this to NO and click save preference.  Get a nice green response.
 

 

 

 

 

BUT it doesn't stick.  The very next time I open a PDF in firefox, up pops Acrobat on my other monitor. 

I try the same action in Chrome and Acrobat does not appear. 
BUT it Does show up when I open a PDF in Edge.

 This redit thread points me to

 Further investigation: Turns out it is Adobe Acrobat's fault. In Acrobat > Menu > Preferences > General > Disable: 'Always open PDFs saved from the web.' So I clear all the tick boxes.

A little further investigations show that the "Always Open PDF's saved from the web" check box gets SET by switching off the PDFs from your device selector.  Hopeless!