First create a new extra field, I'm assuming you have no extra fields yet, so i'm using param00... Open customer/main/product.tpl and insert:
Code:
{literal}
<SCRIPT language=JavaScript1.2>
<!--
function productpdf(param) {
var w = "420"; //width
var h = "420"; //height
window.open(param,"productpdf","width="+w+",height="+h+",resizable,scrollbars=yes");
}
-->
</SCRIPT>
{/literal}
Now add the following button to your product page:

NOTE: If you have extra fields already you'll need to adjust {$product.param00} to reflect the correct extra field with the pdf location you specify below...
I use the files area to upload the PDF files to a newly created directory, then reference the file in the products extra field...
Code:
/xcart/files/pdfs/my.pdf
Simple, but it works... kudos
