Add custom fields to PDF Product Vouchers

functions.php

<?php

// add custom voucher fields - see: https://cl.ly/323U011F0C2u
add_filter( 'wc_pdf_product_vouchers_voucher_fields', 'my_voucher_fields' );

function my_voucher_fields( $fields ) {

  $fields['my_property'] = array(
    'data_type' => 'property',
    'label'     => __( 'My property', 'my-plugin' ),
  );

  $fields['my_number_input'] = array(
    'data_type' => 'user_input',
    'type'      => 'number',
    'label'     => __( 'My number input', 'my-plugin' ),
  );

  return $fields;
}

// return a custom value for a property field. by default, the value will
// be loaded from voucher post meta (get_post_meta( $voucher_id, '_' . $field_id, true ) )
add_filter( 'wc_pdf_product_vouchers_get_my_property', 'my_property_value' );

function my_property_value( $value ) {

  $value = 'my property value';

  return $value;
}

// format a custom voucher field value - see result: https://cl.ly/0v2x073i3B1w
add_filter( 'wc_pdf_product_vouchers_get_my_property_formatted', 'my_property_value_formatted' );

function my_property_value_formatted( $value_formatted ) {

  $value_formatted = '<strong>' . ucfirst( $value_formatted ) . '</strong>';

  return $value_formatted;
}

// To customize the voucher HTML template, copy woocommerce-pdf-product-vouchers/templates/voucher/voucher.php to your 
// theme: my-theme/voucher/voucher.php and customize it to your needs. Note: this template is both by the voucher template
// and the generated vouchers.