At the bottom of the page there's a link to the pdfresurrect package, whose description says "The PDF format allows for previous changes to be retained in a re

At the bottom of the page there's a link to the pdfresurrect package, whose description says "The PDF format allows for previous changes to be retained in a revised version of the document, thereby keeping a running history of revisions to the document. This tool extracts all previous revisions while also producing a summary of changes between revisions."