Foxit Reader
An alternative portable document format (PDF) viewer. Much smaller than Adobe Acrobat Reader, only 1.x MB! Windows only, I think.
UPDATE: 1MB installer is laughably long ago - in 2019 it's 80MB.
http://www.foxitsoftware.com/download.htm
Sumatra PDF
I've seen this referenced a few places. It is open source.
https://www.sumatrapdfreader.org/free-pdf-reader.html