For documents like PDFs, there’s no way to put canonical tags in the web page header simply because there's no site portion. In these types of scenarios, you’ll must use HTTP headers to established canonicals. Ensure that your XML sitemap only features canonical pages. This helps search engines like google https://addons.prestashop.com/it/seo-indicizzazione-naturale/31857-google-rich-snippet-automatico-json-ld-open-graph-seo.html