Maybe you are not permitted to do so.

There are some authors who seem to allow you to download a file. You will see a (well, in my case, blue) button under the article tagged Download, where you can download content or an abstract, whatever they have available in whatever format. In my limited time here so far, I have only downloaded a PDF. Otherwise, you would have to contact the authors for a copy. But I stand to be corrected by those who have worked on the site longer than I have. HTH