Google web search started supporting link rel="canonical" relationships specified in HTTP headers. "As per the syntax described in section 5 of IETF RFC 5988 - webmasters can use rel="canonical" HTTP headers to signal the canonical URL for both HTML documents and other types of content such as PDF files," google revealed.
"A common situation in which rel="canonical" HTTP headers may help is when a website serves the same file from multiple URLs (for example when using a content distribution network) and the webmaster wishes to signal to Google the preferred URL," google explains.
"We currently support these link header elements for web search only. As we see how webmasters are using these elements, we're hoping to add support for them in our other properties," Google stated.
In this example of rel="canonical" HTTP header, a website offering a white paper both as an HTML page and as a downloadable PDF alternative, under following two URLs:
…can signal to Google that the canonical URL for the PDF download is the HTML document by using a rel="canonical" HTTP header:
[Source: Webmaster Centeral blog]