Are you trying to modify the page print version or the PDF print function?
Assuming the latter, and that you're using 1.9.x, and that custom data refers to dynamic form fields, you can simply add
$this->addPage()
to include/class.pdf.php just before
foreach (DynamicFormEntry:($ticket->getId()) as $form) {
Trying to modify print HTML to achieve a page break is possible, but annoying. I've got it working in Firefox if you're interested I'll look it up.