Add Customer Name to Existing PDF At Customer Checkout

I have a PDF book download I sell. I want to provide a personal experience for each person who buys it. I want to swap out a variable such as [FIRST NAME] and [LAST NAME] and add their real first name and last name any place in the PDF where I have those variables set as above. My server is Linux/Apache. I want to do the swap after they return from payment back to my website. I will have their first name and last name as part of the URL when they return to my webpage. For example the return webpage might be [login to view URL] so your scripting would need to grab those variables and create the PDF they will download with all the swaps in the document complete. Please do not bid if you don't communicate FLUENTLY in English. I prefer to use Yahoo Instant Messenger or Skype rather than phone when discussing details. My server is Linux/Apache.