the first place to look is in the copyright section of whatever document you want to post. Most have wording like "this may not be reproduced by any means including electronic, photocopy, written, verbal, smoke signals, etc." If you don't see anything there that prohibits what you want to do to post it, then you're a go. I think a lot of copyrights allow very limited excerpts with proper citation but that's about it. Unfortunately periodicals and just about anything that comes off a commercial printing press is covered by that kind of copyright language. Sorry that this probably isn't the answer you were hoping for.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse