When you solve on an app, you have the arrow keys, the "retry" button, and the engine analysis board tempting you. A PDF is just you, a piece of paper (or a second window), and your brain. You have to calculate 100% visually. This builds deep visualization muscles that apps often atrophy.
A single Mate in 3 problem from a PDF studied for 30 minutes is worth more than 50 blitz puzzles solved in 5 minutes. chess problems pdf
We aren't talking about simple "White to move and win a pawn" puzzles. We are talking about the art of the problem : composed positions where beauty, economy, and logic reign supreme. When you solve on an app, you have