Once the document is published as Public on Issuu, the content will be live and available for everyone to view. The Public Content can not be changed to Unlisted as it is already indexed by various search Engines.
However, if you still want to make your Public content Unlisted, a workaround would be:
1. Download the document which is made Public and delete it from your Issuu account
NOTE: Once the document is deleted, people who have already seen it will no longer be able to access that document.
2. Change the file name of the document you downloaded. (File name becomes the part of the document URL)
3. Upload the downloaded document as Unlisted Content. (This will give the content a different document URL and ID)
NOTE: The statistics and impressions will be removed from the old documents that were Public and will not be carried over to the new documents.