My iPad app of choice for printable puzzles is UPAD, a great little drawing app that works well with PDF files. It is available in both free and paid versions. The paid version, which I use, is $4.99. The Lite version limits the number of puzzles you can store to 5.
Using UPAD, I can store a collection of PDFs on my iPad, and draw on the individual pages using a variety of colors and pen sizes. There is also a highlighter, which is great for marking cells in Slitherlink puzzles. The Undo/Redo function makes it easy to try risky strategies and correct my mistakes. The different colors are great for more advanced Sudoku strategies.
To load a puzzle into UPAD, just visit the Krazydad website (or any other website which offers PDF puzzles, such as The Griddle) in the web browser on your iPad. Click on the puzzle book you wish to start solving. When the puzzle loads, a button will be available on the upper right that says “Open in UPAD”. Click this button. The UPAD app will launch and load the puzzle file. Then you can start drawing on the puzzle pages.
You only need to import the puzzle file once. Later, when you re-launch UPAD, all the puzzles you’ve previously opened will still be there, unless you’ve deleted them.
You’ll find it takes a little trial and error to get used to using UPAD, but once you do, it is very rewarding, and it will save you from wasting ink and paper. The first problem you’ll encounter is that the puzzle is small, and your finger is enormous. You can work around this by zooming into the puzzle, using a “pinch out” gesture. For larger puzzles, I find myself zooming in and out quite a lot.
UPAD is even better with a stylus. You’ll find a number of cheap stylus options on Amazon.
Here’s a tip of the hat to David Millar for introducing me to this cool app.