There are two methods that save changes to PDF documents: the Zend_Pdf::save() and Zend_Pdf::render() methods.

Zend_Pdf::save($filename, $updateOnly = false) saves the PDF document to a file. If $updateOnly is TRUE, then only the new PDF file segment is appended to a file. Otherwise, the file is overwritten.

Zend_Pdf::render($newSegmentOnly = false) returns the PDF document as a string. If $newSegmentOnly is TRUE, then only the new PDF file segment is returned.