It is simple to vertically center things in PDF's, since you have a fixed dimension canvas and text doesn't change dimensions due to user preferences or device

It is simple to vertically center things in PDF's, since you have a fixed dimension canvas and text doesn't change dimensions due to user preferences or device configuration. So if you want layout to be simple, use PDF. Of course most users hate to read PDF's online, because they have to zoom and scroll around and so on. So the real question if you want the pain on the producer side or push it to the consumer side. Producing layout which render reasonably well on completely different devices and screen dimensions is inherently non-trivial.