message: set once: true to add event listeners for buttons of pdf viewer.
  Since these buttons are not recreated after pdf reloaded,
  we do not need to add listeners again.
diff: @@ -350,7 +350,7 @@ document.addEventListener('pagesinit', () => {
  document.getElementById('historyForward').addEventListener('click', () => {
  viewerHistory.forward()
  })
  -});
  +}, { once: true });
  // keyboard bindings
  window.addEventListener('keydown', (evt) => {
  