Microsoft Word files, either .doc or .docx, have become a near universal standard for word processing formats. Fortunately, a Word document can be made mostly accessible quite easily as long as some basic principles are kept in mind.
Before beginning, ask yourself if this particular document needs to be ultimately delivered as a Word doc, or could it be delivered as a more flexible and universal HTML file.
When designing Word documents, use the built-in styles like Heading Level 1, Heading Level 2, etc. to provide a semantic structure to your document. This will help provide a consistent layout and make the document easier to for users of certain assistive technologies to navigate. The default styles in Word can be customized to meet your own needs.
Avoid using text boxes as they make it difficult for screen readers to read the contents of the text box in the proper context of the page.
Be sure to include textual descriptions of images.
When creating lists, use the built-in bulleted or numbered list feature instead of manually inserting asterisks, numbers, or tabs.
In Office 2010 you can use the new built-in Accessibility Checker to check for common accessibility issues. Some of the warnings it gives will be subjective, so discernment must be used in some cases.
Accessibility Cheat Sheet on creating accessible Microsoft Office documents.
