For Change order invoice in Magento Go To this folder app/code/core/Mage/Sales/Model/Order/Pdf/ Here you see 2 files ..

1)Invoice.php :- Please go to line 105 or see the method getPdf()
in this method you are able to call your new function or functionality and it’s functional definition is written on Abstract.php file

2) Abstract.php:- Definition is written on inside getPdf() call function.