A lot of different concepts are getting tossed around in this thread. canonical is one thing. rel is another. <head> and HTTP header are different things.
don't know if you'd canonicalize index.html to "/" since that alone is index, no matter what you name the index page - hope that's phrased right for everyone ...does not compute....
Well, you'd hope that links to "index.html" would never actually occur. But I guess if you've got outside links using this form, you might benefit from using "rel='canonical'" on your internal links.
note that to use this option, you'll need to be able to configure your server
Thanks, google, for that wildly misleading utterance. Honestly, doesn't that make it sound as if you can't issue a header unless you've got control of the config file? Depending on page, you may not even need htaccess.
Adding rel="canonical" to the head section of a page is useful for HTML content, but it can't be used for PDFs and other file types indexed by Google Web Search. In these cases
Again... Doesn't that seem to imply that the http header is only relevant for non-page documents? And how many versions of a pdf have you got, anyway? It's not something you'd ordinarily construct on the fly, except in response to an individual human request.