Physical copies of the Sudarshan Samhita are extremely rare. You are unlikely to find this book in a standard bookstore. Most surviving manuscripts are written in (using Devanagari or Grantha scripts) and are preserved in libraries like the Oriental Research Institute in Mysore or Adyar Library in Chennai. Hence, digitized versions (PDFs) are the only access point for most global devotees.
Downloading ancient scriptures falls under fair use for educational and religious purposes. However, be aware that some modern translations (published after 1950) are still under copyright. If a PDF looks extremely clean and is watermarked by "Motilal Banarsidass" or "Chaukhamba," consider buying the hardcopy to support the preservation of these texts. Piracy of spiritual texts is considered a Brahma-hatya (sin) in traditional Hindu law. sudarshan samhita pdf