By default, mac computers do not add file extensions when you save your files (i.e. ".docx or Word 2007, .xls for Excel). You will need to save your files with the extensions in order to share them with PC users and to print them using Web Print.
To automatically add the file extensions just make sure the "Append file extension" box is checked when you save your files.
Before you share your file or try to print using Web Print it is always a good idea to confirm the file extension is included. The easiest way to make sure it is included is to turn on "show all file extensions" for your computer.