Digital libraries like the Internet Archive host some zoology textbooks by various authors (e.g., N.K. Gupta), but finding a current, complete version of Dr. Ramesh Gupta's text for free and legally is difficult.
No one here can ethically give you a free, full PDF of Ramesh Gupta’s Modern Zoology . If you see a site claiming "free download," it’s likely: Modern Zoology By Ramesh Gupta Pdf Free Download Extra