Sites offering "free PDF repacks" can sometimes bundle malware or phishing links. Stick to reputable community-vetted sources and avoid clicking on flashy "Download Now" buttons that lead to external ad-sites.
For classic literature, search the ISBN of a Penguin Classic edition, then cross-reference with Project Gutenberg. The same text (without critical introductions) is free and legal in multiple formats, including PDF.
The ISBN is a unique numeric commercial book identifier. Originally created in 1966 for the United Kingdom book trade, it has become the global standard for identifying books, allowing publishers, libraries, and retailers to order, list, and track inventory efficiently. In the digital age, the ISBN is often used as a search key to locate specific editions of eBooks, such as PDF files, across various databases. However, this utility has also made it a tool for locating unauthorized copies on peer-to-peer (P2P) networks and shadow libraries.
import isbnlib
Now for the most important section: how to legally download PDFs or other e-book formats using an ISBN, without malware and without stealing from authors.