<?php
function wc_pdf_watermark_change_pdf_password( $order_id, $product_id ) {
	// Use the order number for the password instead of the billing email
	$order = wc_get_order( $order_id );
	return $order->get_order_number();
}
add_filter( 'woocommerce_pdf_watermark_file_password', 'wc_pdf_watermark_change_pdf_password' );