If you’re looking for a (e.g., 5th or 6th), some university repositories legally host them for enrolled students. Check with your campus library.
Oxford prints a low-cost "International Edition" (ISE) which is the as the North American edition, just in paperback. The ISE is often sold as a legal PDF through third-party academic retailers like Abebooks or CampusBooks for as little as $40. --- Microelectronic Circuits Sedra Smith 8th Edition Pdf