It looks like the way HN handles submitted pdf files has changed! Now the title links to the original, and there's a link "[scribd]" after the title if you prefer to view it in the Scribd viewer. Thanks pg!
I'm not sure the tag is voluntary. I submitted it with (pdf) at the end, and edited after the posted title turned into "The First Report on Scheme Revisited (pdf) [scribd] (brics.dk)" which was a bit of a mess.
He doesn't mean the tag is optional, he means he likes how it gives you a choice between just going to the raw pdf and using scribd, so it's a tag making the use of scribd optional.
Better than nothing, but I'd love an option so that all PDF articles into Scribd links in the preferences. Maybe that's just because I'm in the minority of people that likes the Scribd links though. Also, this would allow anyone that never clicks the Scribd links to have a little less clutter on the front page.