>From Firefox, on one PC, a PDF is shown correctly embed in the HTML page, as I have designed it. On another PC, when the user accesses that page, from the same browser, the PDF is sent as a download. Anyone would know what could be in effect here?

Do you have the content type set properly? If not, the browser has no way to handle it and forces it to download.

Example:

            HttpContext.Response.AddHeader("content-disposition", "filename=Certificate.pdf");