There is one small advantage to having a PDF viewer in the browser, but it's a [beneficial] side effect for a missing browser feature.
If you do a google search for something and on the results page is a link to a PDF, the link _isn't_ a direct link to the final PDF file. It's a "result" link that actually points to google (e.g. google?url?sa=t&foo=bar). It redirects when you actually click on it. So, if you right click and select "copy link location", you'll get the link pointing to google and not the final site URL.
For ordinary site links, you just click on the search page link and when you land you have the final link in the page URL, which you can bookmark, copy, etc.
For PDFs, if your action is to run Adobe Reader, it will download the PDF but it loses the sense of the final link. With the embedded viewer, the final link is available in the viewer window's URL, just like an ordinary web page.