Magento – how to change PDF invoice layout

You can customize the invoice layout by modifying this file : Mage_Sales_Model_Order_Pdf_Invoice(app/code/core/Mage/Sales/Model/Order/Pdf/Invoice.php). Of course you need to rewrite the file properly.

Also take a look at these articles for additional info and examples:

1. http://www.atwix.com/magento/customize-magento-pdf-invoice-and-packingslip/
2. http://www.satollo.net/magento-how-to-sligtly-modify-the-invoice-pdf
3. http://www.magentocommerce.com/knowledge-base/entry/custom-pdf-templates/
4. http://www.magentocommerce.com/wiki/5_-_modules_and_development/orders/editing_an_invoice_pdf

You can use:

$pdf = Mage::getModel('sales/order_pdf_invoice')->getPdf(array($invoice));

where $invoice is the invoice object.