Previews are static formats, available for File Types. The standard windows server fonts are installed on our rendering server and will be applied and rendered to document previews. Any other fonts will be substituted. A file format icon based on the default file extension for that file format e.g. Indd for Indesign files - is shown where no preview outputs can be rendered. The original file is also available - accessing (download) the Original File can be restricted with Content Permissions. 

You cannot restrict Access/Download of previews, but it is possible that you add a watermark to Preview which is then also applied to other Formats and Thumbnails derived from the Preview. Check for formats and thumbnails if their rendering source is the Preview format to ensure that the Watermark is applied everywhere.