You can add a border to a PDF document by adding a HTML element that has a fixed position and a border. However it should have no content! This will then be used on every page in your PDF. An example of which is shown below.
<html>
<head>
<style type="text/css">
#pageborder {
position:fixed;
left: 0;
right: 0;
top: 0;
bottom: 0;
border: 2px solid black;
}
</style>
</head>
<body>
<div id="pageborder">
</div>
ABC<br/>
ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>
ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>ABC<br/>
DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>
DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>DEF<br/>
GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>GHI<br/>
</body>
</html>