Microsoft® Word is the world's most popular word processing software application, and files created with Word (typically ending in .DOC) are increasingly common as a means of distributing materials over the web, including materials used in education.
The greatest problem in distributing Word documents is that doing so assumes that all recipients have Microsoft Word. In cases where the entire audience is known, this may be an accurate assumption. For example, in a school where Word is installed on all classroom computers, all students do have access to Word. However, for documents with larger distribution or where users' technology is unknown, some recipients may not have Word and may be unable or unwilling to procure it, since it is not free. In contrast, most web browsers can be downloaded for free, and all of them can display HyperText Markup Language (HTML) documents; therefore, unlike a Word file, an HTML document can potentially be opened by anyone who requests it.
Beyond availability, whether Word is a good choice for accessibility depends largely on the content of the document. A plain text document with no images, no tables, and no headings or subheadings is equally accessible in HTML and Word. Microsoft Word itself is a reasonably accessible application, and many individuals with disabilities use it comfortably on a regular basis to compose their own documents. However, as documents become more complex, they are increasingly likely to present challenges to users of assistive technologies, particularly blind individuals using text-to-speech software (screen readers).
Many states have passed accessible textbook legislation that requires publishers of educational materials to provide their materials in accessible electronic format. In defining accessible electronic format, many of these laws emphasize "structural integrity." An accessible format is one that explicitly communicates a document's structure, including but not limited to headings, subheadings, and table structure. Document structure also allows for features such as alternate text for images, as well as form labels that can be explicitly associated with the fields they represent. All of this is possible with HTML, but only some of it is possible with Microsoft Word.
For example, in HTML, headings can be explictly marked up with a particular heading level: h1, h2, etc. Screen readers have built-in functionlity that enables users to navigate easily through a document's headings, often with a single keystroke. This allows blind users to scan the document before navigating to a particular section of interest. Microsoft Word also provides a heading structure via its styles and formatting functionality. However, most screen readers out of the box are unable to navigate through these headings with the same efficiency as is possible with HTML.
Similarly, HTML provides the ability to add alternate text to images, which is read aloud by screen readers as a way of representing the content of each image. Word provides similar functionality. With most recent versions of Word, there is a field for alternate text in the Image Properties dialog > Web tab. However, most screen readers do not presently read this alternate text within the flow of the document, as they do with HTML. If the author chooses to convert a Word document to HTML or Adobe® PDF, the alternate text is passed on to the converted document. However, within Microsoft Word itself, it presently does little to enhance the document's accessibility.
HTML also provides extensive markup that allows users to explictly define the relationships between table cells and the column and/or row headers that represent them. This is particularly critical for complex tables that contain nested columns or rows. Word provides no similar capability.
Another consideration is that screen readers and other assistive technologies tend to perform poorly when reading a Word document that is opened within a web browser. Much of the specialized functionality that allows users to read, navigate, and better understand a document within the Microsoft Word application is lost when the document is opened within a plug-in. Therefore, in order to experience maximum accessibility, users must download and save the document and then open it in Word. Though not in itself an accessibility problem, this process is burdensome for users.
As these examples demonstrate, in some cases Microsoft could do more to improve the accessibility of Word. However, in other cases, assistive technologies could do more to support the accessibility features that Microsoft has already provided. Authors also share responsibility for accessibility, regardless of whether they are authoring documents in Word or in HTML. Although Word provides a means by which images can be labeled with alternate text, few authors are aware of or use this feature. The same can be said of Word's ability to define various levels of headings using styles.
Given all of these variables, whether Word is an accessible format for distribution of educational documents is not a simple question to answer. It depends largely on the educational context and on the document content and complexity.
Two states have clearly documented their opinions regarding the accessibility of specific file formats within their laws and regulations. In Kentucky, SB 243  (only available as a Microsoft Word document), specifically in its accompanying regulations  (704 KAR 3:455), defines three levels of file format accessibility. The highest (Level 1, or "full compliance") includes XML, XHTML, or HTML format; Microsoft Word falls under Level 2 ("provisional compliance"). Level 3 ("marginal compliance") includes "unlocked PDF."
By comparison, Chapter 219  of New York's Education Laws of 2003 ranks Microsoft Word along with ASCII (plain text) as its fourth and final option, behind full-text DAISY 3 (the preferred file format), HTML (second option), and structured PDF (third option).
Additional information about these and other accessible textbook laws is available in the AccessIT Knowledge Base article Which states have accessible textbook laws and what do they say about file formats? 
-  SB 243
-  accompanying regulations
-  Chapter 219
-  Which states have accessible textbook laws and what do they say about file formats?