In some regions, Klett distributes Schubert materials. They offer institutional licenses. If you are a school director, you can request a multi-user digital license – essentially a legal bulk PDF download.
Some online libraries and bookstores might offer previews or digital versions of textbooks, including teacher's manuals.
Your search for a "top" PDF download suggests you are looking for a digital version. Here is the reality of the digital landscape for this specific book:
Rating for the Book Content: ★★★★★ (5/5) Rating for "PDF Download" Availability: ★★☆☆☆ (2/5) – See "Where to Download" section below.