There are two methods, which provide saving changes to the PDF document. These are Zend_Pdf::save() and
Zend_Pdf::render() methods.
Zend_Pdf::save($filename, $updateOnly = false) method saves document to a file. If $updateOnly is true,
then only new PDF file segment is appended to a file. Otherwise file is overwritten.
Zend_Pdf::render($filename, $updateOnly = false) returns PDF document as a string. If $updateOnly is true,
then only new PDF file segment is returned.