there are several resources where you can potentially find the book "Principles of Quantum Mechanics" by Paul Dirac. Here are a few suggestions:
Library Genesis (LibGen): This website offers a wide range of books, including scientific literature. It provides access to various formats, including PDF. Keep in mind that the availability of specific books can vary.
Project Gutenberg: While Project Gutenberg mainly focuses on providing free access to public domain books, it's worth checking if the book you're looking for is available. However, note that "Principles of Quantum Mechanics" is not likely to be in the public domain yet.
Online repositories and digital libraries: Websites such as arXiv.org, JSTOR, or Google Scholar may have scholarly articles and publications related to quantum mechanics. It's possible that some of them could have the book or excerpts available for free.
Remember to respect copyright laws and ensure that you are accessing the book through legitimate channels. If you have access to a university library or other academic institutions, they may provide online access to the book or have physical copies available.