PDF is a file format and in this context just one big variable since it can be anything. A4, letter etc, too since you can zoom in and out.
It's all about words per line, that's (almost) it.
So, the discussion you're starting in reality is the discussion "I know how many words per line pdfs usually have". And proving that statement right is a huge waste of time, trust me. And that's why I'm not getting into it.
But I can ensure you that I've been absolutely fine reading technical books as PDFs on my 5 inch Sony for a couple of years now. It's fine. Except of that prob that made me open this thread.