[Vlasenko Fedor]

function buildElement(tagName, props) {
  var element = document.createElement(tagName);
  for (var propName in props) element[propName] = props[propName];
  return element;
}

function submit(link, props, callback) {
  var iframe = buildElement('iframe'),
    iframeDoc = iframe.contentDocument || iframe.contentWindow.document,
    form = buildElement('form', {
      method: 'post',
      action: link
    });
  for (var propName in props) form.appendChild(
    buildElement('input', {
      type: 'hidden',
      name: propName,
      value: props[propName]
    })
  );
  iframe.style.display = 'none';
  document.body.appendChild(iframe);
  iframeDoc.body.appendChild(form);
  form.submit();
  if (typeof callback === "function") {
    iframe.onload = function() {
      callback((this.contentDocument || this.contentWindow.document).body.innerHTML);
      document.body.removeChild(iframe);
    };
  } else {
    document.body.removeChild(iframe);
  }
}

btn.onclick = function() {
  submit('php/print_pdf.php', {
    target: 'print_pdf'
  });
  submit('php/print_rtf.php', {
    target: 'print_rtf'
  });
};

сам не пробовал, но походу должно работать