India has a massive market for "free PDFs" of educational and general books. Websites like PDF Drive or Scribd (user-uploaded sections) have normalized the idea that every book should be available as a free PDF. Rangeen Raatein simply falls into this existing habit.
The user performing this search is likely looking for: rangeen+raatein+book+pdf
The reason the keyword "rangeen+raatein+book+pdf" is so popular is rooted in practicality and psychology. India has a massive market for "free PDFs"