if(empty($_POST['content'])){
        exit;
    }
    header("Cache-Control: ");
    header("Content-type: text/plain");
    header('Content-Disposition: attachment; filename="text.txt"');
    echo $_POST['content'];

<div style="display:none">
              <form id="forma" action="add.php" method="post">
                  <input type="hidden" value="text.txt" name="filename">
                  <input id="input_content" type="hidden" value="" name="content">
             </form>
            </div>

function SaveFile(){
    document.getElementById('input_content').value = document.getElementById('блок с нужным текстом').textContent;
    var form = document.getElementById('forma');
  form.submit();
}