As far as I know there is no way to pre-define in PDF initial zoom factor. You may define it as AcroRead command line/URL parameter.
See
http://partners.adobe.com/public/develo ... meters.pdfIn PDF file itself you may define starting document view mode. For example the code
Map m = new HashMap();
m.put(PD4Constants.PD4ML_DOCUMENT_VIEW_MODE, "SinglePage");
pd4ml.setDynamicParams(m);
(or
Pd4Cmd ... -param pd4ml.document.view.mode SinglePage )
forces AcroRead to open the doc scaled down to fit viewer window.