You can do it easily like this:
<GleamTech:DocumentViewer ID="documentViewer" runat="server"
Width="800"
Height="600"
Resizable="True"
ClientLoad="documentViewerLoad" />
<script type="text/javascript">
function documentViewerLoad(sender, e) {
var documentViewer = sender; //sender parameter will be the DocumentViewer instance
documentViewer.setZoomLevel(1); // Set zoom to 100%
}
</script>
Alternative methods that adjust view:
documentViewer.fitPage(); // Adjust page so that both width and height fits the view
documentViewer.fitWidth(); // Default behaviour