If you open a PDF with more than one revisions in the documents section you will see the compare button on the top right of the document next to the revision dropdown.
When you click the compare button the current revision will be compared to the revision before it.
Clicking the compare button again cancels the comparison.
When you click on the compare dropdown you will have the option to choose what color the current revision has and what color the revision you are comparing to will have.
The slider lets you adjust the opacity of which revision you want to show more clearly.
Annotations with compared documents
It is possible to make annotations of two compared pdf revisions.
This annotation can be stored in the comment section of an issue like a regular annotation. You will be able to see if an annotation is made on a comparison between two revisions in the issue by the revision number which will be displayed as: #1/#2