Custom Fields at checkout on PDF

Hello,
I am new to this and am after some help I have the Pdf Invoice and packing slips plugin. I would like to get 2 custom fields i have at checkout to show of the PDF invoice that gets sent to my cusromers. However i have no idea where to start to do this or if it is even possible. Ewout helped me last year with a custom template for Printing address labels and i am hoping you may be able to help again with this. Kind Regards Nicole