It's caused when you call documentViewer.setZoomLevel function in documentViewerLoad event when viewing the page in mobile mode (via Chrome Device Toolbar). As a workaround call setZoomLevel function only when not in mobile mode:

<script type="text/javascript">
    function documentViewerLoad(sender, e) {
        var documentViewer = sender; //sender parameter will be the DocumentViewer instance

        if (documentViewer.selectedType != "html5Mobile")
            documentViewer.setZoomLevel(1); // Set zoom to 100%
    }
</script>