First off, I loved the first Puzzle Agent. Completed it in one sitting and was satisfied by it. The puzzles were hit or miss, but they were fun enough even when they were easy.
So far, I love the second game as much. The writing is still pretty good and the mood is unique for a video game.
But that space digits puzzle felt like a complete mistake. I was pretty good at math in high school, but I don't know more than four pi digits. The thing is that I understood the puzzle, but I couldn't do it and had to look elsewhere.
Like I said, I love the mood of the game, and "alt-tabing" the game to check the number on internet destroyed the that mood. I quit the game and came back later.
Also, it's pretty misleading that the number are in pair. It's like they want you to think about something else (because the puzzle would be pretty easy if it wasn't for that).
I think in games like this (like Puzzle Agent and Prof. Layton), the player should be able to solve the puzzle when he understands it. Also, they shouldn't be hard because they're misleading, they should be because they present a logical challenge.
I don't think the answer is common knowledge, but maybe I'm wrong. I still love the game and think it has an unique mood for a videogame. Sorry for the rant.