That one is a scanned PDF; its contents don't appear to be indexed by, e.g., Google; its availability is dependent on the continued goodwill of the ACM.
Paul Graham's copy is plain HTML, sitting on the open web where anyone can see it and index it, and while for the rest of us its availability is dependent on PG's continued goodwill presumably he likes being able to rely on it staying there as long as he wants unless he's explicitly forced to take it down somehow.
You have a point. I still find it odd to use a personal domain (other than the author's) for this purpose, with the Internet Archive and other options being available.
Searching for "Computer Programming as an Art" the PDF comes up as the second link (below PG's rip off). Not that it's even relevant because it's given as a direct link.
A scanned PDF is a closer reproduction to the original presentation, and easier to save for later to read.
I trust the ACM's "goodwill" to continue hosting it more than I trust it to remain on PG's website.
Paul Graham's copy is plain HTML, sitting on the open web where anyone can see it and index it, and while for the rest of us its availability is dependent on PG's continued goodwill presumably he likes being able to rely on it staying there as long as he wants unless he's explicitly forced to take it down somehow.