Hi @AndyHoward,
Canvas does not have the abiltiy to check documents created outside of the LMS and then uploaded into it. You would need to use the tools that are available in the software (such as Microsoft Word) or online-based resource (such as Google Docs) where the document was originally created.
If you are creating something in Canvas while using the Rich Content Editor, you can use their "Accessibility Checker". More information about that can be found in their "How do I use the Accessibility Checker in the Rich Content Editor?" support article at https://community.canvaslms.com/t5/Canvas-Basics-Guide/How-do-I-use-the-Accessibility-Checker-in-the....
More information about Canvas' accessibility standards can be found at https://community.canvaslms.com/t5/Canvas-Basics-Guide/What-are-the-Canvas-accessibility-standards/t....
Your own institution may have licenses to other third-party resources that integrate with your institution's Canvas environment to help with accessibility compliance but you would need to check with your institution's Canvas team for help with them.
-Doug