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/developer/en/acrobat/PDFOpenParameters.pdf

In PDF file itself you may define starting document view mode. For example the code

[language=java:3sx4er5j]Map m = new HashMap();
m.put(PD4Constants.PD4ML_DOCUMENT_VIEW_MODE, “SinglePage”);

Pd4Cmd … -param pd4ml.document.view.mode SinglePage )

forces AcroRead to open the doc scaled down to fit viewer window.