Adobe Acrobat Reader Dc

I've had this read for quite a while now and never had any issues downloading PDF's and seeing them.

But for some reason as form this week they have stopped opening and I keep being displayed this error message ' This file format cannot be converted to PDF using drag and drop or right click Menu.To convert, launch Acrobat and go to File->Create'

I am trying to open the same files as I did before and haven't changed anything on my MacBook air.

Can anyone help?