Ghostscript, and derivatives (gv for Unix/Linux systems, GSView for MS Windows) is an extremely useful utility for viewing both postscript and PDF documents, as well as converting from these formats to a variety of other formats or printers. Ghostscript will handle most PDF files, but unfortunately cannot replace Adobe's Acrobat Reader program entirely, because some PDF files will confuse it.
Ghostscript is available from http://www.cs.wisc.edu/~ghost/.