SteveOH

To get around this issue and delete the document, you must first Un-Publish the document, and then you can Delete the document, without an issue.

BUT, this will not delete the File.  For clarification, the file is the actual physical file that sites on the server, the document is the listing with a title, description, permissions and associated file.

You then must delete the file from the back-end.  This functionality is intentional, as you can link multiple documents to a single file.