Can I just say, it can be a little fiddly.
Http:// has to be replaced with file:/// and have to be in " "
If you open the Operation_Manual.pdf in your browser, and copy the URL, then paste it replacing the URL in the help-redirect-EN.html (or whatever language you use) remember the " "
Here is how it looks on my system:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Steinberg.help: Are you offline?</title>
<script lang="javascript">
function init ()
{
var queryString;
if (navigator.onLine)
queryString = "file:///C:/Program%20Files/Steinberg/Cubase%209/Documentation/Operation_Manual.pdf"
else
queryString = "offline.html"
window.document.location = queryString;
}
</script>
</head>
<body onLoad="init();">
</body>
</html>