A single Mate in 3 problem from a PDF studied for 30 minutes is worth more than 50 blitz puzzles solved in 5 minutes.
You can curate a specific collection. Want only Mate in 2 problems by Soviet composers from the 1950s? There is a PDF for that. Want Miniatures (7 pieces or less) to solve during your coffee break? There is a PDF for that. You own the file forever; you aren't renting it from a subscription service. chess problems pdf
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
Search for "A Century of Chess Problems by Alain C. White (PDF)". It is public domain, contains 404 masterpieces, and if you solve ten of them this week, I promise you will see the board differently in your next tournament game. There is a PDF for that