If the specific arrangement or author of the 66 Wechselspiele has entered the public domain (typically works published before 1928 in the USA), IMSLP is the gold standard. Users often upload "high quality" scanned versions from university libraries. Use the search filters to find "600dpi" or "monochrome" scans.

If you own a physical copy, the best way to guarantee a high-quality PDF is to do it yourself. Use a flatbed scanner at 300 DPI (600 DPI for detailed fingerings) and save as a searchable PDF. This gives you ultimate control over the darkness, contrast, and cropping.

Perhaps the most famous. The pianist plays a motif in the middle register, then immediately leaps two octaves higher with the opposite hand. Blurry scans make the ledger lines look like spider legs; quality scans make them unambiguous.

High-quality editions of the Wechselspiele include dynamic markings that change per hand. In a poor scan, a "p" for the left hand and an "f" for the right hand might blend together. In a high-quality scan, the contrast and resolution preserve these vital nuances.