Firefox和Chrome都内置了PDF浏览器，挺方便的。但有时候我想直接下载PDF，而不是默认由浏览器打开。此时可以通过设置来关闭它们。如下引自Lifehacker:
The built-in PDF viewers in the Chrome web browser and Mozilla Firefox are convenient because you don’t need to install another program to view PDFs. However, if you prefer using an external PDF reader, you can disable the built-in browser viewers.
In Firefox, go to Options > Applications, then find “Portable Document Format (PDF)” and click on the drop-down menu to choose an option other than Firefox.
In Chrome, enter, “chrome:plugins” in the address bar, then click the Disable link under the “Chrome PDF Viewer” plugin entry.