Loading php file with AJAX in WordPress frontend

Try using get_template_part() function. use ob_end_clean() to Clean the output buffer and turn off output buffering. I have use html() to append data jQuery('#ajax').html(response.content);. I have tested and working fine for me.

jquery :

jQuery("#ajaxbtn").click(function () {
    jQuery.ajax({
      type : 'post',
      dataType : 'json',
      url : myajax.ajax_url,
      data : {action: 'tablo'},
      success: function(response) {
        jQuery('#ajax').html(response.content);
      }
    });
  });

ajax function :

add_action('wp_ajax_tablo', 'tablo');
add_action('wp_ajax_nopriv_tablo', 'tablo');
function tablo() {
  // Grab php file output from server
  ob_start();
  get_template_part( 'ajax' );
  $result = ob_get_contents();
  ob_end_clean();
  $return = array('content' => $result);
  wp_send_json($return);
  wp_die();
}