Alan Bleiweiss: Yeah since it is a PDF, there is no header tag within the doc. Instead you add the canonical to the header through the server, such as in the .htaccess file.
Alan Bleiweiss: https://moz.com/blog/how-to-advanced-relcanonical-http-headers
Michael Martinez: It`s optional. Google suggests you wrap the document in HTTP headers if you can do that but doesn`t require that you do. https://support.google.com/webmasters/answer/139066?hl=en
If you have a PDF download of an article that you have on your site and both are indexed. Should on have a canonical pointing to the other? 
