Opera ditched the built-in PDF viewer in Chrome and opted for Mozilla's open source one
The new Opera 15 may be based on Chrome, but the browser maker isn't copying all of the features – sometimes by choice, sometimes because it can't. One area where Opera is striking out on its own is the PDF viewer.Google Chrome comes with a built-in PDF viewer, which is a plugin that ships with the browser and runs in the sandbox. It's built by Google, unlike the bundled Flash plugin, but not with web technologies.
However, Opera chose to borrow Firefox's much more web-friendly approach and package PDF.js, the PDF viewer in Firefox, as an extension for Opera.
The PDF.js extension, dubbed simply PDF Viewer, is published by Opera itself. Since PDF.js is open source, as Mozilla intended all along, Opera was able to grab the code and simply package it as an Opera extension.