That's fairly simple. Just upload the PDF file to your server, then create a link to it in the product description.
Your link will look similar to this:
Code:
<a href="PATH_TO_PDF_FOLDER/PDF_FILE.pdf" target="_blank">Click here for more info on this product</a>
Change the stuff in all caps to the location of your PDF file in relation to your cart folder. For example, if your cart folder is in http://mysite.com/zencart/ and your PDF file named data.pdf is in http://mysite.com/pdf/ your relative link in your item description would be "../pdf/data.pdf"
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