You can't embed a PDF in an HTML file, that is physically impossible. At most you could display it in an iFrame, but I really wouldn't recommend that, because users would have terrible display problems in the small frame. It is really better to just link to it, or get hold of the original content and integrate it in your topics, if you can.
HTML files can be embedded in WebHelp with the #MERGE command. See here for details:http://www.helpandmanual.com/help/index ... gehtml.htm