TheOptimizationIdiot - 8:53 pm on Apr 10, 2013 (gmt 0)
canonical is one thing. rel is another.
You're really confusing the issue for people, imo.
rel and canonical and href go together in a <link> in the head of the page or in the HTTP header.
Link Rel in the <head>
<link rel="canonical" href="http://www.example.com/page.html">
Link Rel in the HTTP Header
Link: <http://www.example.com/page.html>; rel="canonical"
The concept is canonicalization.
There's nothing different being "tossed around" here.
you might benefit from using "rel='canonical'" on your internal links.
It would do no good to put rel="canonical" on links. It Must go in the <head> section of the document or the HTTP header to be recognized. Anywhere else and it gets completely ignored.
Doesn't that seem to imply that the http header is only relevant for non-page documents?
No. You're reading too much into things and really not understanding rel=canonical.
It was designed from the start to be flexible and easy to use.
You can put it in the <head> or HTTP header. Either is fine. Putting it anywhere else will have no effect whatsoever, otherwise people could drop a link here (or anywhere that allows links) with rel=canonical pointing to their site and effectively hijack the page.
And how many versions of a pdf have you got, anyway?
It's not about how many version of the pdf you have it's about having an HTML page and a pdf (or plain text print version or xml/rss version or other HTML version) with the same information and being able to say which version is preferred for search engines to show in the results.