Why does Orwell present the book in this way instead of paraphrasing it or treating it as a
appendix?

Orwell includes Goldstein's book as a separate entity to the main plot, with a purpose of challenging the Party and its philosophy.

Explanation:

1984 is a dystopian novel from 1949, written by George Orwell. It is about a man called Winston, who is a low ranking member of 'the Party'. Winston is frustrated by the Big Brother, a man who controls people's lives.

in the novel, Goldstein's book ( "The Theory and Practice of Oligarchical Collectivism") functions as a separate entity to the main plot. The book serves as a contradictory book which opposes the Party and its philosophy and represents a socialist utopia.