Andy_Langton - 9:50 pm on Feb 27, 2013 (gmt 0)

I think you're confusing the desktop with the web somewhat. On a PC, there are "file associations" whereby particular applications open particular file types (which are known by their extensions). There are no file extensions or file associations on the web. The browser determines what to with a particular response by the content-type header, so if you want to name an HTML file .pdf, you can, as long as you serve it as text/html. Whether browsers download or display PDFs depends primarily on what PDF software is installed on an end-user's machine.