The key is the second parameter (40) of the function setMargins() in writePage() in PDFGenerator.php
40 is the top-margin of the main content (body). This is the space ocuped by the header. Reduce it to 0 to remove the header
public function writePage() { ... $this->setMargins(10, 40, 10);
In versions <1.8 this parameter was 20.
In versions >=1.8 this parameter is 40