Google Books offers several titles from the Ulysses Moore series in English. The advantage here is that you can read directly in your browser (which prints to PDF) or download an ACSM file that converts to PDF via Adobe Digital Editions.
The series is presented as a collection of diaries found by the "translator," adding a layer of meta-fiction that makes the stakes feel real. For English-speaking fans, the series is primarily known by its first book title, The Door to Time. Why Readers Search for the English PDF
If you are looking for a free but legal option, your local library may have digital copies. Using the app, you can borrow the e-book and read it on any device. While you won't own the PDF permanently, you can read it for 14–21 days for free.