I have experience converting PDF documents to HTML. If I understand you correctly, you want the information in the PDF to be listed underneath the product description, right? This conversion process is very easy to perform.
Zen Cart is very customizable. I have built many Zen Cart based websites over the last 18 years and have made many modifications to them based on the webmasters needs, so adding a link to a PDF is easy to do as well.
What kind of additional fields do you need added to your product description page?
Feel free to contact me and let me know exactly what you need. By the way, I am available for immediate hire.