It's complicated, and while I'm not a lawyer, I'll try and explain what I've learned over the years in dealing with this issue online...
1. You're welcomed to make a copy of any article you find online and save a copy to your computer for your own personal use. In fact, every time you visit a webpage, a copy is already being automatically made in your Web browser's cache. So if you read a newspaper article are are afraid it will "disappear" (it's usually moved into a pay-only archive), that's a good way to keep a copy for your own personal use.
2. Emailing a single copy of an article to a single person is something that is permissible under copyright laws.
These kinds of two uses above are covered under a legal doctrine called "Fair Use." Fair Use basically says you may make copies of copyrighted material for your own personal use or research. Emailing to a single friend (or 3 or 4 friends) is an extension of this kind of use that some websites allow because they hope it will drive more traffic back to their website someday. That means you're also allowed to share an article via PM here to a few friends.
However, when we post something here to NeuroTalk on the forums, we're no longer making a copy just for our own use or sharing it with a single friend. We're making it available to everybody on the Internet and it's no longer "personal use" (and may or may not be covered by the Fair Use doctrine).
If you post a copyrighted work here for "educational or research purposes," it may be allowed under Fair Use. But we'd greatly prefer that whenever possible, you simply link to where you found the article online, because that is completely allowed and won't ever get us into trouble (whereas posting copyrighted works here may).
PS - If you want to learn more about copyright in a relatively easy-to-understand format, I like this page: