There are 10 coupons in a mail. When somebody wants to print one, they will click a link in the mail which will generate a unique, printable (PDF) coupon code for that person.
How to generate that coupon code - where will the script reside? Will there be a server listening to the clicks?
The link in the email should go to your site.
Something like the following: Email -> yoursite.com/coupon.php?generate=1
coupon.php will be the php script on your server generating the coupon. The coupon can either be pre-generated and displayed or generated on the spot.