At the bottom of DocView.aspx (located by default in Program Files\Laserfiche\WebLink\Web Files\), you should see code that looks like:

        <script type="text/javascript">                    
            $(document).ready(function() {
                $($get("MetadataTab")).click();
            });
        </script>

You can switch the "MetadataTab" to either "ThumbnailsTab" or "AnnotationsTab" to set which will be the default tab. Make sure you set the actual code (seen above), and not the comment right above it marked by <%-- and --%>.