That book is fantastic, one of my favorites. It's been a couple of years since I last read it, and your comment reminded me that I should probably skim again.
Except that PDF shouldn't exist since it was generated in flagrant disregard for the copyright/comment:
% This manual is copyright (C) 1984 by the American Mathematical Society.
% All rights are reserved!
% The file is distributed only for people to see its examples of TeX input,
% not for use in the preparation of books like The TeXbook.
% Permission for any other use of this file must be obtained in writing
% from the copyright holder and also from the publisher (Addison-Wesley).
\loop\iftrue
\errmessage{This manual is copyrighted and should not be TeXed}\repeat
If you want a free PDF on TeX try _TeX for the Impatient_:
https://tug.ctan.org/info/impatient/book.pdf
If you want a copy of _The TeXbook_, buy it.