The Portable Document Format is one of the more ubiquitous document types both on the Web and on personal devices, and can be used for distributing manuals, brochures, and most other formatted documents.
However, until now Firefox, one of the more popular Web browsers, in part due to its availability on OS X, Windows, and Linux, has not had a built-in PDF reader. Instead, to open PDFs it's been necessary to either install a browser plug-in or download the PDFs and open them in a managing program like Apple's Preview.
However, in the latest version of Firefox, Mozilla has included a native PDF viewer that ought to make possible quick and complete viewing of PDFs without the need for other programs and plug-ins.
In Firefox 19, released this morning, if you open a PDF either from a Web link or from your hard drive, it will appear in the browser in a simple but efficient interface that has options for zooming (manually or automatically), viewing a PDF in a full-screen presentation mode, navigating to specific pages by number, and scrolling through thumbnails of the loaded document.
If you are a Firefox user then chances are that you have Adobe's Reader plug-in installed, so if after updating to version 19 you still see the same Adobe PDF interface, try going to the Firefox Add-Ons manager (in the Tools menu) and clicking the option to disable the Adobe Reader plug-in.
While the new reader is a great option, it does lack one feature for Mac users: support for gestures like pinch-zooming, which can be extraordinarily useful for quickly viewing small PDF content. However, a native PDF viewer in the browser is a very welcome and long-awaited addition.
Firefox 19 should automatically download sometime today when you run Firefox, but you can also download the latest version from the Mozilla Web page.