Office 365 and some prior versions have a built-in Accessibility Checker. This can be found in some versions under File > Info > Check for Issues. This will identify a limited number of issues. See Microsoft's Accessibility Checker help page for more information.
Accessible Word documents start with using appropriate styles. You should also include appropriate alternative text for images. In the current version of Word for Windows, right-click on the image and go to "Format Picture". Under "layout and properties", the third icon, expand the Alt Text. Enter in the description field for the alternative text. A title is not required but is helpful. (The description field is read by the greatest number of screen readers and is what is exported as the alt text when creating a PDF.)
WebAim has full details on how to create an accessible Microsoft Word document.
Word for Mac
You will need the most recent version of Word for the Mac to generate a tagged PDF from a word document. Prior versions will not create the tags needed for a PDF document to be accessible, requiring much additional work in the PDF to generate valid tags.
The built in accessibility check will highlight several of the items below and allow you to directly fix those items.
Use the built-in layouts, as these are connected to the outline. If you delete the text fields on the layout and add your own, the text you enter will not be accessible to screen reader users.
General accessibility of powerpoint documents follows the accessibility general principles. Images and other such content like charts and tables should have alternative text. Prior to distribution, you should remove animations and transitions. Embedded audio and video should have multimedia text equivalents. You should also ensure there is a logical reading order. The color contrast for text should be sufficient; you can use the feature to render in greyscale as a quick test.
Webaim's PowerPoint Accessibility provides additional step-by-step instructions on making MS Powerpoint presentations accessible.