How so? I have no problem opening PDF links directly in Safari.

Safari "supports PDF", but not well. The PDF viewer is run in an extremely janky view that clearly has not been updated in years. It runs out-of-process, but takes little advantage of the many advances in XPC rendering that have come in recent years; as such it cannot handle looking up services correctly, or vibrancy, or even have Retina support for its UI. And those are just visual: the PDF support itself is shoddy; it's unable to do many things that other browsers can do out-of-the-box (forms?), searching for text has been broken for the better part of a year. It's an obvious rough spot in Safari's otherwise polished interface.

Fair criticism. I guess my bar for what I define as "good PDF support" is much lower than yours - I only generally read them or plug in a digital signature when signing my lease.

Personally, I am loath to download documents. I actually like what iOS Safari does, which is run the generic document previewer on files inside the app itself. I hate clicking on a link on my computer and then getting a PPTX that I have to open in PowerPoint.