It sounds like you're referring to the popular piano method book (likely by Éditions Hervé or similar French publishers), and you’re asking about an interesting feature of its PDF version.
However, there's a : This specific book (Volume 1A) is still under copyright in most countries (France, EU, etc.). Therefore, a free, public PDF is unlikely to be legal . If you found one online, it’s probably an unauthorized scan. The interesting "feature" of such scans is that they often have missing pages, skewed images, or incorrect fingerings compared to the original printed edition. les classiques favoris du piano 1a pdf
Here’s the key point: