Custom title when using shortcode

This code solved my problem.

function digitSix()
{
      //some codes here;

      $var = "this is custom title";

      add_filter( 'pre_get_document_title', function( $var ) use ( $var ) {
            return $var;
        }, 20 );

      return $someContent;
}

add_shortcode("i_digit6", "digitSix");