functions.php

<?php
/**
 * Save a upload field.
 */
function my_save_pdf_uploads( $post_id ) {
  $field = $form->find('pdf');
  $field->save_uploads();
  
  foreach( $field->uploaded_files as $filename => $id ) {
    update_post_meta( $post_id, 'my_custom_pdf', $id );
  }
}
add_action( 'ap_processed_new_question', 'my_save_pdf_uploads' );