Hi, I just did similar project on prestashop site. I just want to let you know there are some limitations in pretashop invoice.
Prestashop is using well-known free PDF class called FPDF which generates PDF files using PHP. However, there is a problem with the way Prestashop handles pdf invoices. It is possible to write the module that will change the default look of invoices, but there are only small number of things that you can do, in fact. You can, for example, add text to the bottom of invoice. But it is not possible to modify the design of invoices from Prestashop module. There are no so-called hooks for this. Of course, I can edit the core prestashop files and this way I can change the invoices layout any way you wish - but changes will get lost when you upgrade presatshop. So it will be necessary to backup the appropriate files, and, after upgrade, upload the files back to prestashop.
So this is the limitation. I suppose it may take me about 3-4 days completing the project and total of $150. This is not very difficult task but needs some tweaks. I hope my bid is really reasonable.
Thanks for considering my bid. Patrik