You have two versions of the same content in HTML (on the website and in PDF). What is the best solution to bringing a user to the site with the full navigation instead of just downloading a PDF file?

The X-robots rel=canonical header is the best solution because it tells search engines that the HTML version is the preferred, canonical version of the content. This directs users to the website with full navigation, rather than the PDF. Introducing hreflang using X-Robots headers is incorrect because hreflang is used for specifying language and regional targeting, not for canonicalization. Using the X-robots-tag and the noindex attribute is incorrect because noindex would prevent the PDF from being indexed at all, which might not be the desired outcome if you still want the PDF to be discoverable in some contexts, and it doesn't solve the canonicalization problem of preferring the HTML version.