This software is available for almost every OS and lets you view PS and PDF files.
Web-site: http://www.cs.wisc.edu/~ghost/
PDF browser, available for MS Windows and Linux.
Web-site: http://www.adobe.com/products/acrobat/readermain.html
PDF browser, available for MS Windows and Linux.
Web-site: http://www.foolabs.com/xpdf/
In many Linux distributions it is possible to view PS files under X Window with gv, PDF files can be viewed with xpdf.