: Covers light, optics, relativity, and quantum mechanics.
The most significant driver of the PDF search is availability. The original two-volume and three-volume sets published by W. W. Norton & Company are largely out of print. While used hardcovers exist, they are often expensive or worn. Consequently, students turn to digital copies to access the material without paying collector prices.
If you find a copy (digital or physical), Volume 3 is a gem for understanding the philosophy behind quantum spin and time dilation, not just the formulas.