What's wrong with the SimpleImage User's Guide? When I try to view it, Acrobat Reader displays an error message on just about every page.

Version 4.5 of StuffIt Expander has a bug which can corrupt PDF files if the "Convert text files to Macintosh format" option is set to anything except "Never."

Try setting this option to "Never" in your copy of Expander and decompressing the .sit file again.