Puzzles of Sudoku Free Edition has the same gameplay as the paid version, but has the following differences:

 

• Puzzles are easier to solve, even on the maximum level of difficulty;

• Undo/redo information has a limit on the moves that are tracked (hopefully a big one, but still a limit) and it's not persisted if you quit the app;

• Banners are shown while playing (no banners on the paid version).