Create modify, the invoice template in a open source php program http://www.activecollab.com. activeCollab uses FPDF PHP library to generate PDF files, so you'll need some PHP knowledge to be able to tweak existing code and make it work with our design, font, etc....
Jason,
This PDF/PHP work is pretty straight forward, I'm new to freelancer and need to boost my ratings hence the low bid, I've sent a message to you with a link that will show you an example of PHP/PDF