How do I save as Word document?

Pages is not a Word replacement. If you want a product that saves Word documents, you are better off with MS Word (which costs more) or NeoOffice/OpenOffice (which cost nothing).

Pages can export to Word's doc format, but it can be annoying to always have to select and overwrite the document you already have with a new version every time you change. Besides the export is far from perfect.

If you are set on using Pages, and nothing but Pages to edit your Word documents there is a hack, which is described at leefindlow.com. That site describes very well how to do it, so I will just add a short summary here for the record.

Please, remember that Apple never intended us to do this, and anything you do slightly wrong may mess up Pages for you. Only follow the following steps if you know exactly what you are doing.

1. Right-click (or ctrl-click) on the application icon for Pages and select "Show Package Contents" from the pop-up menu.
2. Find the file Contents/info.plist. (You may want to make a backup copy of it, in case you mess up in the following steps.)
3. Open the file in an editor like Property List Editor. If you are comfortable with XML, you can also use a text editor like TextEdit or TextWrangler.
4. Look in CFBundleDocumentTypes/Document types. There you will find a list of different Document types (CFBundleTypeName).
5. Find the document type Microsoft Word document.
6. Check its associated Role (CFBundleTypeRole). By default it is set to Viewer.
7. Change the Role to Editor.
8. Find the document type Microsoft Word 97 - 2004 document.
9. Check its associated Role (CFBundleTypeRole). By default it is set to Viewer.
10. Change the Role to Editor.
11. Save the file info.plist.

From now on, when you open a Word document in Pages 3 or 4, it will be able to save the file when you simply press cmd-S.