PDF Online is an online service that has both fee and FREE services relating to PDF files.
One of the free services is "Doc2PDF" that will convert a document to a PDF file. You can only convert files up to 2GB (to convert files up to 10GB you need to subscribe to the premium version).
This is good for people who don't convert many files to PDF. If you want to convert a lot of files to PDF, I suggest CutePDF.
The other free service is "PDF to Word" which converts your PDF documents to RTF Documents for viewing and editing Word. It works very well and fast. After uploading your file, it will convert it and then leave a link for you to download with the converted file.
Another way to do this is with PDF2Word.
PDF files are very handy for sharing files and posting files online. The ability to create them or edit them is very useful for teachers.