The most well-known free PDF readers, which both offer a browser plug-in, are Adobe Reader and Foxit Reader.
The open source
The open-source Sumatra PDF reader is another alternative. Note: you'll have to enable the installation of the plug-in during the setup.
The plugin-free add-on
For newer versions of Firefox, an add-on which doesn't require the installation of plug-ins is also available. It's called PDF Viewer and is based upon the PDF.JS project.
It is currently available for Firefox for Android 11.0 - 16.0a1, Firefox 10.0 and later and SeaMonkey 2.7 and later.
Note: PDF plug-ins such as Adobe Reader can conflict with the add-on. You'll have to disable these via Ctrl + Shift + A > Plug-ins.