This is designed to make the browser more secure, since it eliminates the need for third-party components that often come with proprietary source code Mozilla has no control over, and which could pose security risks.
PDF.js did not create any problems during the six beta releases for Firefox 19 but should something happen on its account, Mozilla has already prepared an add-on hotfix to disable the component.
Important to note is that, when PDF.js is disabled, the previous PDF plug-ins become re-enabled. The hotfix affects only the release channel.
For Windows, there is also a stub-installer available, which downloads the necessary files over a secure connection.