I'm really stuck with this problem...
I believe you can set the zoom level in the document itself and the embedded PDF will honor that setting but I haven't tried it myself.
You would need Acrobat to do this and you would find it in File>Properties>inital view>Layout and magnification
Thanks for all your answers so far guys.
@George Johnson: The URL parameters don't seem to be working anymore. I tried them before but none of them work. The document you given me is also dated from 2005.
Any other suggestions?