So I heard an interesting question the other day, one that's worth thinking out loud about. Someone asked whether it was legal, copyrightly-speaking, to post a legally open-access article to a public server or service (such as Facebook or FriendFeed), or if one should link instead.
The answer, as with most copyright questions, is "it depends." The other answer is "I am not a lawyer; if you have a copyright question, go ask a lawyer." But in my estimation, even when reposting is probably safe, I think it's better to link, and I'll try to explain why I think that.
First, there's a pragmatic argument: it's usually just plain easier to drop in a link than to download and reupload (and if it isn't easier, the hosting archive is broken). I'm all in favor of easy.
Second, in many cases, reposting articles publicly may well infringe copyright. If there's a CC-BY license on the article, I would guess public reposting with credit to be an acceptable reuse. If there's a CC non-commercial or share-alike license, I'd personally think twice. If there's no CC license at all, which is the usual case? By reposting, you're making a copy, and yes, an author or copyright-owning publisher could bring a lawsuit over that. Would they have much of a case? Who knows? I don't. But who needs the hassle?
Can I, as a digital-archive manager, give you permission to repost items from the archive I run? Actually, no, I usually can't (the few CC-BY items in the archive aside). The license that archive depositors give the archive lets the archive disseminate materials via its own website. That license emphatically does not let the archive give other people permission to disseminate (except perhaps under the specific circumstance of the archive shutting down and transferring the entirety of its assets elsewhere). It's a subtle point, but important.
Third, there's an impact question to consider. As alternative impact metrics take hold in journal publishing, view and download numbers take on new importance for authors. If you repost an article instead of linking to it, are you going to count views and downloads? Probably not. Publishers and archives, though, they're counting and reporting. So anybody who downloads your copy robs the author of a countable download. Maybe that doesn't matter much today… but it might matter a lot tomorrow.
Fourth, authors aren't the only folks counting views and downloads. Digital archives aren't magically free to run, and we digital archivists don't work entirely out of the goodness of our hearts. One of the ways we justify our work and our archives' existence is through view-and-download counts. When you repost, you dilute the impact that we can report to our funders. Speaking as one whose service has been threatened with closure—any impact dilution can be a true threat.
Link, don't repost, even when reposting is legal. The author you benefit may be your colleague, or even yourself. The open-access archive or publisher you benefit is fighting against the paywall-bounded darkness.