A buddy of mine set up a little internet thing that turns documents into Adobe Postscript (.pdf) files. I have tried it with Word docs and it works great. I think it will work with Excel spreadsheets and Powerpoint presentations as well.
His idea was to give away a free trial .pdf document that would have a watermark on it. Then he would charge a small fee for the "real" version without the watermark.
However, he is giving away the service for free while he tests it out! Give it a try and see if you like it.