Here is a list of the best download sites for free pdf ebooks. Some sites will require an email address to download the books but most will not.
The Open Library
The goal of the Open Library is to make published works of humankind available to everyone in the world. They provide ebooks to read online and to download using a number of formats including PDF.
Bookyards has thousands of free PDF ebooks on education materials, information, reference materials, and more.
Pdf Drive is a search engine for PDF books and files. They have links to millions of eBooks for you to download for free.
Obooko is a unique collection of free books available in pdf, Kindle, and ePub format.
B-OK is a search engine for PDFs and other ebook formats where you can search the largest free online libraries in the world. There are literally millions of pdfs available to download here.
This site features a huge collection of free pdf ebooks and tutorials in categories such as Computers, Lecture Notes, Mathematics, Tutorials, Programming, and Technical books.
This site lists free PDFs eBooks and online books related to programming, computer science, software engineering, web design, mobile app development, networking, databases, information technology, AI, graphics, and computer hardware.
Alice and Books
This site free PDF ebook editions of public domain literature and distributes them free of cost.
PDFBooksWorld publishes digitized PDF versions of books that are in the public domain in the most popular and generic PDF format which can be read on many devices.
This site offers free computer science, engineering, and programming ebooks, texts, and lecture notes, in PDF format all of which are legally available to download.