Possible to open/convert Office 98 for Mac PowerPoint file
There are a bunch of free online converters out there that will convert many different file types to PDF.  I had to convert some Microsoft Works documents to Work once and the zamar.com worked great.  Try one of these out:

http://www.freepdfconvert.com

http://www.zamzar.com

http://document.online-convert.com/convert-to-pdf

Let me know if any of these help you out.  Good luck!