Sumatra PDF - Free PDF Reader

Sumatra PDF is a free PDF, eBook (ePub, Mobi), XPS, DjVu, CHM, Comic Book (CBZ and CBR) reader for Windows. Sumatra PDF is powerful, small, portable and starts up very fast.

Also, it doesn't lock the PDF it shows in the filesystem and auto-refreshes when the PDF changes. Quite useful when previewing files generated with Asciidoctor and the like.