Acrobat has long included a browser plugin for IE and Netscape. I believe you don't need to do anything fancy (no OBJECT tag) to trigger it; it's automatically a "helper app" for any .pdf files. The effect of the plugin is to "take over" the browser client area.
Similarly, I believe that IE will open Word (if available) as a helper app for .doc files. Considering how often Word is a delivery vehicle for macro virii, this is terrible behavior that may not be default on all systems. I'd assume the latest version of IE wouldn't do it, but you never know with
Trusted Computing.

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