Displaying PDFs directly in the browser would definitely improve the user's experience. There're literally millions (billions?) of PDFs floating around the web, and on many devices loading PDFs switches to a different application (e.g. Preview on OS X and PDF View on Android).
Mozilla team have been developing pdf.js the code on GitHub albeit quietly, for about a month now. "We intend to use pdf.js to render PDFs "natively", within Firefox itself. Our most immediate goal is to implement the most commonly used PDF features so we can render a large majority of the PDFs found on the web," revealed the Mozilla.
"Initially we'll make a Firefox extension available to interested users that enables inline PDF rendering using pdf.js, but our ultimate goal is of course shipping pdf.js with Firefox. This'll result in a substantial usability but also security improvement for our users. pdf.js uses only safe web languages and doesn't contain any native code pieces attackers could exploit. Additionally, we want pdf.js to be a community driven and governed open-source project," Mozilla said.