How to Change the path of a child page?

Question

$mypages = get_pages( array(
    'parent'     => get_the_ID(),
    'sort_column'  => 'ID',
    'sort_order'   => 'ASC',
    );

foreach( $mypages as $page ) {      
          // Modify this line to include ID of page where want to change the link 
      if ($page->ID == $ID_OF_PAGE_THAT_SHOULD_BE_PDF_LINK) {
          // Modify this line to include your PDF link
          echo '<li><a href="https://wordpress.stackexchange.com/questions/154920/YOUR_PDF_LINK_HERE">' . $page->post_title . '</a></li>';

      }
      else {
          echo '<li><a href="' . get_page_link( $page->ID ) .'">' . $page->post_title . '</a></li>';  }

     }