The rule of thumb is, if it's in your course, it needs to be accessible. So if you include links to documents, those documents must be formatted for accessibility. Please see Making "External" Docs Accessible.
All Microsoft Suite applications (Word, PowerPoint, Excel) have built-in accessibility checkers. Please see the Microsoft Accessibility Checkers page for more details.
Google provides some general guidelines on making your document more accessibleLinks to an external site.. Also, the Chrome browser offers an extension called "Grackle Docs" that checks for accessibility issues in Google Docs, Google Slides and Google Sheets. You can find it in the Chrome Web Store.
Check at least three documents of each type (Word, PDF, Google) to get a sense of whether appropriate formatting is present (all the same stuff as on a Canvas page: headings, lists, descriptive links, table headings, alt text, color contrast/meaning). If not, train the instructor on proper formatting techniques for documents. (Perhaps have a conversation with them about the benefits of bringing content directly onto a Canvas page. It supports students in not having to click/download and supports the instructor by making the RCE available for ease of formatting.) If just an errant issue or two are found, inform the instructor of the problems so correction can be made.
If the document was created using MS Word, use the built-in accessibility checker. Please see the Microsoft Accessibility Checkers page for more details.
GrackleLinks to an external site. is a web add-on that can be used to check accessibility of Google applications. If your college has a GSuite account, they can add on unlimited user access across your college domain for a single group license. Generally speaking, you need to have Edit rights of the instructor's Google document/slides in order to run Grackle on their content.