Using Cdn For Static Contents And Google Ranking

Using CDN can definitely increase your Google Ranking, but it also might have a slightly bad effect on it, if you don’t tell Google that the contents you’re linking to from CDN are “canonical”. Google is not human, and it won’t always realize that http://cdn.site.com/file.jpg is an OK content, and it does belong to your site, so they announced official support rel=”canonical” attribute within HTTP headers.

This enables webmasters to set up a canonical for linking to or from PDF files or other non-HTML based files.
One of the examples given by Google is that a “webmaster can signal to Google that the canonical URL for the PDF download is the HTML document by using a rel=”canonical” HTTP header when the PDF file is requested.”

Another 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.
More details can be found on the Google Webmaster Official Blog.