On FileSearchBox, you can search the entire web for every public-facing PDF available! That means that if a website offers a .pdf download without you needing to login or anything, then you can access it through this search engine.
Every time you want to learn something, be educated about a hobby or field of study, or get deeper content on a topic, this is the best place to search for PDFs. You see, blog posts and web content is great for learning, but nothing beats having an entire, comprehensive book on a topic. By searching for a topic, you can often find ebooks available that cover the topic in much greater detail than you’d find in a blog post.
This is especially true of fields of study. For instance, if you want to learn statistics, you probably won’t find many resources just by searching a standard Google search. By comparison, you may even find free textbooks when you search for PDF files.
Technically yes, some of the PDFs this search engine finds are books. If somebody uploads a book to a public server, and doesn’t exclude the PDF file type from being indexed, Google will index that book.
Whether or not they give you rights to download the book is hard to say. I am not a lawyer, and cannot recommend your course of action here.
Since we’re using a Google Custom Search to find PDFs, we are beholden to whatever Google is allowing to happen on their search engine.
I would say, at a minimum, that you should not distribute any free books you find using this search engine unless the book says otherwise, or the author has given permission to do so.
Do you like learning? Are you frequently looking for more information? Make sure you bookmark this site so that any time you want deeper information about a topic you can come search for a PDF.