The physical book is spiral bound so it lies flat. However, this makes the spine fragile. When the book breaks, students scan it to PDF. The PDF is actually more durable than the original.
You're looking for information on "Nihongo 500 Mon N3 PDF"! nihongo 500 mon n3 pdf
: Frequently hosts the full PDF files for the entire "Shin Nihongo 500 Mon" series, including N1 through N5. The physical book is spiral bound so it lies flat