MS Edge for Android can enable its own PDF viewer in some versions. Otherwise you probably cannot view a PDF file on your smartphone web browser – e.g. Chrome or Firefox. You would need to download it first.
On this site
PDF icons are hidden on mobile phones, as are PDF text links.
If a download button appears faded then the website thinks that you have downloaded that file. It will not stop you downloading it again, though.
Some PDF files are too big for this website to download. As a result, whole paragraphs can be left out on mobiles.
You would also need to ensure that a PDF reader is installed on your phone. Acrobat, developed by Adobe, is one of the best and the reader is free. The editor is not. The Adobe website is the safest place to get it.
If the button stops working try clearing your browser cache. Better not to clear cookies though.