Many universities and educational foundations now publish "Open Textbooks." These are books written by experts and peer-reviewed, available specifically for free download. Searching for "Open Textbook Mathematics" or "Recurso Educacional Aberto Matemática" can lead you to high-quality, modern arithmetic textbooks.
If you are looking for a deep dive into the history and "verified" heritage of arithmetic, here is the story of where these books come from and which ones are most significant. 1. The Heritage of Classic Brazilian Arithmetic livro de aritmetica pdf verified