Tax Invoice Redesign

I have a Creloaded Oscommerce website which can produce tax invoices ether by me through the admin control panel or by the customer via there login area. I would like Tax invoice form reformatted. 90% of this job is cosmedic (visual changes only) I need a way of manually inputing a customer Purchase Order Number into a predefined space I will supply two pdfs, the first shows what is being produced now, the 2nd shows the way I want it to look. Please note that the heading "Tax Invoice No:" Replaces the heading "Print Order #:" also that the address/contact information is entered through the admin control panel.