Whether you own a PC or a Macintosh, the procedure is the same. Beside it is a button with a downward-pointing triangle. Clicking on it brings a pop-up menu of file formats in which your document can be saved. Very similar dialogues exist on the Mac version and in other applications, including WordPerfect and AppleWorks. Just look for the pop-up list of formats.
Choices are wonderful, but confusing.
This is as close as possible to a universal format. This is almost as universal as plain text, these days. Even if a person's word processor can't display it properly, their web browser will. Microsoft developed this format specifically for exchanging documents with their formatting between programs, and most modern word processors support it. With foresight and luck, you can share information with people who use different programs on different computers.