Sumatra PDF is a free PDF, XPS, DjVu, CBZ and CBR reader for Windows.
Sumatra PDF is small and starts up very fast.
Simplicity of the user interface has a high priority.

• Sumatra has built-in support for easy previewing of LaTeX documents. Read this page for more information and detailed instructions on how to configure various text editors to integrate with Sumatra's preview functionality.
• The "full installer" version includes a browser plugin for Firefox, Chrome and Opera (Internet Explorer is not supported). It's not installed by default so you have to use installer's options button to install it. You might need to configure the browser too: enable Sumatra's PDF plugin and disable PDF plugin from other programs.
• A restricted mode is useful if you want to use SumatraPDF as a bundled viewer for your program's documentation or in kiosk mode.
• If "Remember settings for each document" is turned on and a PDF is later opened again, Sumatra PDF will return to the page shown when it was closed. Settings are not only stored for those 10 files shown in the "File" menu - but resuming will not work if the file has been moved in the meantime.
• Some settings (e.g. fullscreen) are saved as defaults without a GUI option, when SumatraPDF is closed without any document open.
• Editing interactive forms and adding comments is not implemented.