You would think no one would accidentally leave their file server open to the world. Yet, these directories are everywhere for three main reasons:
If you need a 500-page textbook, small PDFs are useless. Search by file size. intitle index of pdf books
intitle:"index of" physics textbook pdf "1.5 MB" OR "2 MB" OR "5 MB"
Note: This is less reliable on Google. A better method is to find any directory and sort by "Size" descending on the directory page itself. You would think no one would accidentally leave
The inurl: operator searches for keywords in the URL itself. Combining it with intitle: is incredibly powerful. Note: This is less reliable on Google
intitle:"index of" (pdf|epub) inurl:library
intitle:"index of" inurl:ebooks
This finds directories where the URL contains words like "library" or "ebooks", increasing the chance of a high-quality collection.