the link does not point to a pdf, it points to an abstract
Furthermore, depending on publishing site, a paper may also be available as HTML rendered from the LaTeX source, in addition to PDF. (If the page does not now, it may in the future.)
The purpose of a [PDF] tag is to warn about possible unsuitability of the linked resource for mobile consumption (which isn’t the case for the article page linked here), possible download size (though maybe not anymore, nowadays), and possible brightness shock when using dark mode.
On the few occasions I post submissions like this I specifically and deliberately, where possible, post links to abstracts, not the actual papers. People can then skim the abstract and decide whether or not to go further.