I've not found a way to do this from within the JavaScript of a form...I don't know that it's possible. I have looked at this issue before though, specifically regarding opening Dynamic PDFs in a SharePoint webpart. If you have a URL to a .pdf file, you can add #toolbar=0 to hide the toolbar. I believe there are couple other things you can add to the URL as well.
Here's a linke to where I found this:
http://www.planetpdf.com/forumarchive/115936.asp
Hope this helps!
Ryan D. Lunka
Cardinal Solutions Group
rlunka@cardinalsolutions.com