I just installed Rockbox on an old iPod.
The installer has an option to download the manual in PDF, which works, and as a zipped HTML, which is a 404.
The website manual page:
http://www.rockbox.org/manual.shtmlis the same: links to PDF work, links to "online" are all dead.
One reason I'd prefer the HTML is that in the PDF crossreferences ("page XX") are coloured blue, but aren't actually links. Presumably they work in the HTML.