A hash would still only work for the exact document. Suppose someone uploads a new version with a better table of contents, or with some added annotations [0];

A hash would still only work for the exact document. Suppose someone uploads a new version with a better table of contents, or with some added annotations [0]; now your hash no longer matches, while contentID probably will. [0] Both things which could be copywrited, but I'll assume will not trigger the actual flag.