The PDF represents not piracy, but a search for signal in a noisy educational landscape. If you find a legal copy, treasure it. If you use a scanned copy to learn, consider buying a used hardcopy when possible or donating to an open-access mathematics initiative.
C. L. Liu’s second edition is more than a textbook. It is a quiet mentor, waiting to teach you how to think precisely about discrete structures—one relation, one graph, one recurrence at a time. Elements Of Discrete Mathematics Second Edition C L Liu Pdf
The connection to algorithm analysis (Big-O, divide-and-conquer) is implicit but clear. The PDF represents not piracy, but a search