For files like PDFs, there’s no way to position canonical tags while in the web page header simply because there's no website page area. In these kinds of conditions, you’ll ought to use HTTP headers to established canonicals. For those who’re syndicating or republishing material on other web sites, this https://addons.prestashop.com/fr/seo-referencement-naturel/31857-seo-automatique-google-rich-snippets-json-ld-open-graph.html