Honestly, that's because PDF is not well-thought-out.
Choosing to use PDF to distribute text (or text-and-images) on the Web today does seem like a pretty silly idea. PDF favors exact reproduction of a paper-based layout over readability on the user's device; and that's the opposite of what's useful if you're trying to get a message across to many users.
But, given that PDF is out there, it's pretty useful for a mobile device to be able to deal with it competently.
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: