hi,
I think the plugin thing is controlled by the settings in acrobat itself. Yesterday i was messing with some font properties and I went into the general properties and i found the checkboxes there.
But if you say you are getting the plugin in some browsers and not others in the SAME computer, then I guess it's the browser's problem or maybe there is a way in the PHP headers to invoke the plugin or specify the content-type as a plugin/pdf or something like that.
A quick and dirty way is to load a blank HTML file and in that use an EMBED or OBJECT tag and specify the src to be the dynamic .php file, and specify the plugin-type or content-type to be PDF in the embed/object tags. I've never done this before and I'll be happy to try it out later today if needed. Just post a message or drop me a note at: php@theslinky.com
-sridhar