I think the difference is that WeTransfer requires you download the pdf before viewing it, whereas this site lets you view the pdf in-browser (and page 1 can be viewed before the whole thing is downloaded).
That is a feature of modern browsers, but the mime type has to be set right for the browser to not treat it like a download.
That is a feature of modern browsers, but the mime type has to be set right for the browser to not treat it like a download.