S. Chand does not officially release a free PDF. However, you can legally access a digital version via:
The Revised Edition (S. Chand, 2010 or later) is the best because it includes SI units and updated constants. Avoid the 1960s editions. Atomic Physics J B Rajam Pdf
The book is generally divided into two highly detailed sections: SapnaOnline Part 1: The Beginnings of Atomic Physics Atomic Physics J B Rajam Pdf