I need to replace the invoice id to order id on my invoice PDF (see image). Spent a few hours researching without finding a proper guide on getting started with customising magento's PDF :/ and explored a few extensions but i really don't wanna add an extension for this. So any help would be greatly appreciated.
The most simple way to do this is to overwrite the core PDF model for invoices.
First copy the file
Now open the new file and replace the following line
Mage::helper('sales')->__('Invoice # ') . $invoice->getIncrementId()
Mage::helper('sales')->__('Order # ') . $order->getId()