add_action( 'gform_after_submission_3', 'download_pdf_after_submission', 10, 2 );

function download_pdf_after_submission( $entry, $form ) {
    // Change 'your_acf_field_key' to the actual ACF field key where the PDF file is uploaded
    $pdf_url = get_field( 'upload_pdf', $entry['post_id'] );
    // If PDF URL is not empty and is a valid URL
    if ( ! empty( $pdf_url ) && filter_var( $pdf_url, FILTER_VALIDATE_URL ) !== false ) {
      // Redirect the user to the PDF URL
      echo '<script>window.open("' . esc_url( $pdf_url ) . '", "_blank");</script>';
  }
}