Although it is certainly possible to edit previously-created documents to make them accessible, the easiest way to ensure documents are accessible is to build them that way from the beginning. Once you decide to make all documents accessible, build new ones using the accessibility standards and work through old documents by starting with the ones that are least accessible (i.e. need the most work to become accessible)--since those would be the least useful to someone using adaptive technology.
Even if a Microsoft Word document only has text in it (no images or links), there are still steps to take to make it truly accessible for someone using a screen reader. Word has built-in features that work with the screen reader's software to improve the experience of the user who is unable to see the visual organization--when those features are used in the creation of documents. (Pre-existing documents can be edited to use the features, but it is easiest to build documents from the beginning.)