Many of us would need customized PDF Slips, Invoices, (I mean more customization than just changing the logo and shop name).
I suggest to use PDF Forms for it. So every shop owner could design and generate their fancy PDF Template files both for the Invoices and the Slips using Adobe Acrobat or any other PDF Generator program, upload it to a directory, and Nopcommerce would fill out only the required pdf fields programmatically with data when generating Invoices, Slips, etc.
The library iTextSharp is capable of filling out existing PDF Forms, so not to much change in code is required. This would also be a more clear programming approach (separation of concerns), the design of these PDF Prints would not be hard-coded into the program any more.