PDFBooksWorld is described as 'Free pdf books on fiction, non-fiction, academic, textbooks and children's books available for download in high quality PDF format' and is an website in the News & Books category. There are three alternatives to PDFBooksWorld, not only websites but also apps for SaaS and Windows. The best alternative is Project Gutenberg, which is free. Other great sites and apps similar to PDFBooksWorld are College Student Textbooks (Freemium) and Centless Books (Free).
Project Gutenberg is a library of over 60,000 free eBooks. Choose among free epub and Kindle eBooks, download them or read them online. You will find the world's great literature here, with focus on older works for which U.S. copyright has expired.