I think Scribd is a Y-combinator funded startup (hence the Hacker News connection) and some of the Hacker News admins by-hand edit other people's posts. I am very against copying PDFs into a walled-garden like Scribd, but this isn't my website.
One is simply using a service, the other involves rehosting content without the request or permission of the author or content owner.
As far as I know, they don't do that, but if they did, how would it be different from Google cache?
That said, it's irrelevant because they don't crawl for content. The HN admins are way out of line with their practices on this one. They're making illegal copies to help their friends at Scribd, and doing so without the requisite consent.
See also: http://docs.google.com/viewer
That said, two points:
1) doing so is not just immoral, it's against Scribd's TOS.
2) your claim that re-hosting content without permission is indistinguishable from transport makes clear that your beliefs are so different than mine that I cannot possibly find a way to communicate with you.
There may be case law and/or actual laws that clarify these points, but I am not a lawyer. I presume you aren't either. If you think the concept of IP law has a straightforward and indisputable application to the internet that clears all questions about what routers can and can't do with the data they are passed, feel free to explain.
It seems like you're operating within the "lots of people do it, so it must be legal somehow" school of thought when it comes to routing. This isn't necessarily a problem as long as it's applied consistently. Several other commenters and I effectively made the same argument in saying that Google does almost the same thing Scribd does in terms of copying and redistributing content. It isn't possible for you to call that argument invalid then rely on that argument as proof of routing's obvious legality.
Scribd is legally indistinguishable from a caching proxy. Feel free to let Opera and all the other caching proxy operators know.
You already said you weren't a lawyer. No need to prove it dramatically.
robots.txt is indeed intended for crawling, but if it's there and you redistribute someone's content anyway, I'd consider it less defensible.
I'm pretty sure that one of the purposes of the site is to attract people to YC. If you want to know why people like myself (who are just here for the hacker news/discussion) stick around, it's because it has some of the most interesting and civil discussion of any online community, and if you know about the YC bias then it's not a big deal to filter it out in your mind when browsing/reading the site.
I tried Foxit and was not very impressed. I thought it was interesting that the Evince windows port was made since the last time I looked for an Acrobat replacement about a year ago.
I think the rationale is that some people don't like PDFs. They don't want to download something when they're just going to skim it now and probably never look at it again. I don't really get it, but I guess it would make sense for very long PDFs so that you don't have to wait to download the whole thing when you might take one look at it and decide you don't care.