Here are some links that provide some very handy information on changing the Magento pdf invoice layout.

http://www.xpcdesign.com/2008/12/edit-the-magento-invoice-pdf-templates/
http://www.magentocommerce.com/wiki/how-to/editing_an_invoice_pdf

The default invoice font size is very small, our client was having trouble reading his invoices! I managed to increase its size quite easily as indicated in the first link, but could only increase it a bit before the invoice layout started to break (I would need to alter the layout a bit to allow for bigger font sizes, hopefully this won't be necessary!)