How to change the default filename when using mPDF

$payStub=new mPDF();

$payStub->SetTitle('My title');
$payStub->WriteHTML($pcTableRows);
$payStub->Output('yourFileName.pdf', 'I');

1. 'D': download the PDF file
2. 'I': serves in-line to the browser
3. 'S': returns the PDF document as a string
4. 'F': save as file $file_out