And in 2016 still no answer. I was just strugling my self with this and after a while, success. I dont know which Prestashop was released in 2012 but, I can give a few hints for latest version of prestashop which is on todays day 1.6.1.9 (my current version).
For anyone who is trying to modify invoice in prestashop, you'll find the template file in folder pdf and file name is invoice.product-tab.tpl for editing and adding fields concerning products (prices, tax, discounts, etc.)
All you need to know is this:
$order_detail.product_name -> returns the name of the product
$order_detail.unit_price_tax_excl_before_specific_price -> returns the price of product without tax and before discount
$order_detail.unit_price_tax_excl_including_ecotax -> returns product price without tax and included eco tax (if there is 1)
$order_detail.order_detail_tax_label -> return the tax percentage
$order_detail.product_quantity -> returns the quantity of orderd product
$order_detail.total_price_tax_incl_including_ecotax - > returns the total amout that is product price + tax
With this codes you can calculate discount in precentage or in cash and you can use it in teplate. There are few more other codes in .tpl, but thats not part of this question.
Hope this will help anyone.