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.
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.
Sure, if you search for its title. But if you search for text within it, you'll find it doesn't come up at all.
> I trust the ACM's "goodwill" [...]
Sure. My suggestion is that Paul Graham trusts his own goodwill more than the ACM's and that that's one reason why he's put a copy on his website.
What I find truly astonishing is that this 1974 paper is so relavent today.
Computer Programming as an Art
By Donald E. Knuth
It's that simple.