We can add watermarks in advanced PDF templates as Images and texts. We can rotate these watermarks also. Please see the below code snippet for the reference.
<head>
<macrolist>
<macro id="watermark">
<p rotate="-45" font-size="48pt">
Confidential
</p>
</macro>
</macrolist>
</head>
<body background-macro="watermark">
<p><!--Content--></p>
</body>