Okay, here's what you can accomplish with just the default AbanteCart offerings.
1. In your product page - add your PDF file for that product only - on the media tab. You will have to decide what settings you want for your customer to have access - immediately, on completion of order, etc. Those are business decisions.
that will give you the file for each product, instructions , labels , whatever you need to tell them for that operation.
2. then on the admin panel System > Localization > Language Definitions find the one for the checkout_success block - contains the wording on that page -
<p>Your order has been successfully processed!</p><p>You can view your order history by going to the <a href="%s">My Account</a> page and by clicking on <a href="%s">History</a>.</p><p>Please direct any questions you have to the <a href="%s">store owner</a>.</p><p>Thanks for shopping with us online!</p>
you can change it to whatever you need it to say and to direct your customer back to there account for the downloads.
Note there are two of them - one for accounts, one for guests if you allow guest checkouts you will need to change both
See if this will get you what you want. By making them into downloads on their account they can reprint them if necessary.
Best thing I could think of at the moment. Let us know if it will do it for you, or if I need to keep thinking.
Lee