Bing often indexes directories that Google has removed due to DMCA requests. Try: intitle:"index of" "pdf books" on Bing. DuckDuckGo (using !g bang) is less effective but respects privacy.
Short Guide To Indexing | PDF | Spreadsheet | Books - Scribd intitle index of pdf books
The query intitle:index.of pdf books is a relic of the early 2000s—a time when the internet was less commercialized and security was an afterthought. While it still works today, it represents the "wild west" of file sharing. Bing often indexes directories that Google has removed
Look for /pdf/ or /ebooks/ in the URL path: intitle index of pdf books