As a matter of convention around here, which is considered most appropriate? A link to an outside website, or copy-paste the whole article into a post?
If it's an article on your own web site, then it's best to just copy and paste the whole article into your message. But if it's someone else's article on a third party web site, it's usually best to post a paragraph or two of the article and post a link to the rest of it. That way, people can at least get an idea of what the article is about before they jump over to a completely different site to read the rest of it.