A massive, free alternative on GitHub that covers many of the same core principles.
I understand you're looking for a review of a PDF version of System Design Interview – An Insider’s Guide by Alex Xu. However, I must note that distributing or obtaining copyrighted technical books for free (without the author’s or publisher’s permission) is generally illegal and violates the author’s rights. Alex Xu’s book is commercially published and protected by copyright. System Design Interview By Alex Hu Pdf Free