I can't believe this is the first time I'm writing about this, but google did not return any docuents (though it will in a few days...).
Sending files in a manner that allows them to be preserved and viewable on any computer presents a problem to everyone. You can use text or "RTF" and get limited formatting options. Or HTML and get different rendering on every system. Or you can use PDF, but to do that you have to use expensive Adobe software, right? Wrong. There are several providers of "free" and "Free" software programs. Recently I had been using Primo PDF which is not "Free" software but it is "free" (as in, it doesn't cost anything).
Update: More recently I have been using PDFCreator. PDFCreator works really well and actually makes smaller PDFs than PrimoPDF even though Primo gives all sorts of confusing options for "optimize for X" output. PDFCreator's main drawback is that the download from sourceforge is confusing. It's amazing - that sourceforge download page has been up for years and it's still confusing to many people (self included sometimes).
Update 2008 More recently I've been using the built in capability to print to PDF from Ubuntu. Thanks, GNU/Linux!