If something is copyrighted (everything is, these days), you may quote a piece of it as long as that piece cannot stand alone or is not a substantial part of the whole (in other words, no more than 10% of the original piece). Otherwise, you must have written permission, and in many cases, pay a licensing fee for use. This applies whether it is in print or on the Internet - it is really no different.
As an example, in one of my recent articles, I wanted to use a table from the ESC Guidelines. To do so, I first had to get written permission from the chair of the anticoagulant section of the guidelines (Prof. Lip) and the chair of whole guidelines committee (Prof. Camm) -- fortunately, both are on our medical advisory board, making it easier. Then, we had to get permission from the European Society of Cardiology, because they own the guidelines. Once that was done, we had to get permission (again in writing) from Oxford University Press as they publish the journal in which the guidelines were published. Fortunately we were able to get it all done in a few days, thanks to help from Prof. Camm and Prof. Lip. But the cost of publishing without written permission from all parties is much too great.
Publishing a whole article, or significant portions of it, on this forum puts the person who posted it and the publisher of this forum (StopAfib.org) at significant financial and legal liability. As an author myself, I am very sensitive about having my own intellectual property compromised, and I cannot let this happen to someone else's intellectual property.
Rather than taking a shortcut of just copying and pasting, we have to ask you to provide the title and link, and preferably to state what the article is about or why someone should bother to click on the link. Thanks.