Select as default PDF handler button greyed out

[larmstrong7979]

In adobe reader DC the "Select as default PDF handler" button is greyed out even when logged in as administrator.  how do i enable this?  i need this endabled as i also have Adobe Acrobat Reader DC installed (on RD Servers) and our case management system will ignore the default progam for PDF and try and use acrobat DC instead of reader DC and the only way to fix this is to press that button as administrator.  does anyone know how to resolve this or do they know exactly what the button does apart from set the file association ie add a registry entry?

thanks