You need to follow the steps to open the file in the viewer, for which you need to compare revisions.
Next, you can follow the steps to navigate to File Viewer More Options.
In the 'More Options' menu, select the 'Compare File With' option (under the 'More Info' category) to compare the file revision open in the viewer with its other revisions accessible to you. Click 'Compare File With' and select the previous revision from the auto-populating list. The files will open in the comparison viewer.
Click here to understand how to compare files in the comparison viewer.
Next Article: View Workflow Instances on a File Open in Viewer