An example of how to create and use a randomized voucher number like 4ST0P-TGIBC-ZBBGU-VNQ8A with the WooCommerce PDF Product Vouchers plugin

wc-pdf-product-vouchers-custom-voucher-number.php

<?php

// Create and use a randomized voucher number
function serial_p4u( $number, $voucher ) {

	// if we've already generated a custom voucher number, get it and return it
	if ( $voucher->get_item_id() ) {
		$custom_serial = wc_get_order_item_meta( $voucher->get_item_id(), '_voucher_custom_number', true );
		if ( $custom_serial ) {
			return $custom_serial;
		}
	}

	// otherwise, generate one
	$tokens = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
	$serial = '';

	for ( $i = 0; $i < 4; $i++ ) {
		for ( $j = 0; $j < 5; $j++ ) {
			$serial .= $tokens[ rand( 0, 35 ) ];
		}

		if ( $i < 3 ) {
			$serial .= '-';
		}
	}

	if ( $voucher->get_item_id() ) {
		wc_update_order_item_meta( $voucher->get_item_id(), '_voucher_custom_number', $serial );
	}

	return $serial;
}
add_filter('woocommerce_voucher_number', 'serial_p4u', 10, 2);

Hi Justin,
Can you give me some hints at which places I may tweak your code to work with wc 3.x/ wc ppv 3.x
Thank you in advance.
Eric