Print pdf with vertical centering contents in Reveal.js. Run this snippet on the Chrome dev tools just before exporting the PDF. (https://github.com/hakimel/reveal.js/issues/563)

vertical_centering.js

function toArray(o) {
  return Array.prototype.slice.call(o);
}

toArray( document.querySelectorAll( '.reveal .slides section' ) ).forEach(function(slide){
  var outerWrapper = document.createElement('div');
  outerWrapper.style.cssText = 'display:flex; align-items:center; height:100%';

  var wrapper = document.createElement('div');
  wrapper.style.width = '100%';
  outerWrapper.appendChild(wrapper);

  toArray( slide.children ).forEach(wrapper.appendChild, wrapper);

  slide.style.height = '100%';
  slide.appendChild(outerWrapper);
});