You find the style sheet in a kind of pseudo css file called invoice.style-tab.tpl in the pdf directory. Or you modify directly the invoice.addresses-tab.tpl.
Best way for changes is not to modify directly in the original directory, but to copy the files to a new directory pdf in your theme dir. Otherwise your changes will be lost after the next upgrade ( which hopefully comes). You can create here as many invoice templates as you like, even as a single file, as long as you rename them (must be different from invoice). Afterwards choose your active invoice template in /orders/invoices. Same drop down like for invoice and invoice-B2B.