Disable the option to edit SP document library in Desktop Word Application

I want to change my document libraries in Sp Online to allow staff to view, download and open in Word Desktop App, however I do not want them to be able to edit the document once it opens in the Word Desktop Application — only to Save As another name on their PC. I want to prevent them from altering the form in SP in any way. What do I do? Thx. Caulene