Hi Jacopo, the version comparison you are referring to does not work for uploaded documents. Re-uploading a document simply overwrites the old document. To use the version comparison, you will need to create the document natively in Jive (create a document, write or copy & paste content). When you make changes to that native document, you can use the version comparison to see edits that have been done each time the document has been in edit mode.
Hope that makes sense!