Google is moving beyond the ancient NPAPI platform for plugins in Chrome. Firefox is doing pretty much the same, though it's not dropping support completely like Chrome is doing. Overall, leaving behind the old platform is a good thing, but there are cases where this creates a problem.
For example, if you have disabled the built-in PDF reader in Chrome, you may have already noticed that PDF files open outside the browser, regardless of whether you have a PDF reader with a browser plugin, like Acrobat Reader, on your computer.
"If you disabled the native Chrome PDF Viewer in Chrome Canary from chrome://plugins, opening a PDF is now disabled by default even if you've installed the last version of Adobe Reader on Windows," Google's François Beaufort revealed.
While Chrome hasn't left the NPAPI plugin platform entirely behind yet, so it could in theory run the third-party PDF reader plugins, Google is blocking those already.
What this means is, if you want to read a PDF inside your browser, you either use the built-in plugin or you're out of luck.
PDF.js may not be as compatible or as feature-rich as the Acrobat Reader plugin or even the one built into Chrome, but it's certainly a lot safer and, for the most part, faster than those.