Unfortunately it seems the PDF library used doesn't provide that functionality. You may need to come up with your own solution, like adding _blank to the submit query. See https://stackoverflow.com/questions/896724/how-to-open-a-new-window-on-form-submit.
The list of options the library provides can be found at https://github.com/moodle/moodle/blob/master/lib/pdflib.php#L183.