Cool PDF Reader is the smallest and Free PDF reader that I know. As it is small (only 650KB), the software can be opened very quickly. Apart from its size, Cool PDF Reader is more than a PDF reader, it can:
-Convert PDF to graphic such as BMP and JPG
-Extract PDF to TXT
-Support many different language
-Zoom and rotate PDF page
-Slide show PDF document
Moreover, portable version of Cool PDF reader is available. This is the best for office which user does not have administrative right to install software.
My comment is that this software cannot compare with the Adobe PDF reader in terms of functionality. However, Cool PDF reader is best use for the computer that does not have much system resources and office use. I wonder why Adobe PDF reader is so large, as 650KB is enough for performing most of the common functions.