Is it possible, and how if it is, to have different header for pdf invoice and pdf credit memo.
In other words is it possible to have separate Abstarct.php. One for pdf invoice and other for pdf credit memo.
You could just implement the
_drawHeader function in the respective Invoice/CreditMemo subclass and not in the Abstract class...
As always, make sure to create a module for it and do not edit it in the core :-)
By the way, you may want to have a look at FireGento_Pdf, which already provides nice-looking PDFs and may also be a good starting point for your custom PDF rendering functionality.