Hi ,
Could you please tell the OS you are working on?
Ideally the save option is disabled from Reader DC.
If we talk regarding Acrobat DC ,you could try and repair Acrobat if you are on windows and then see if that helps.
If save does not work for you ,you could use save as ,as both have the same functionality however save as would give you a dialog box to modify the name and location of the PDF.
Are you able to save the PDF by using the keyboard shortcut?
Try rebooting your machine as well .
Regards
Sukrit Dhingra