Embedded files and objects are not supported in the exported PDF. They output as an image in the PDF and are no longer accessible. For example, an embedded Excel table in the PDF document is then an image, as a result Assistive Technologies can no longer read the contents of the table.
Embedded files and objects such as Excel files are not supported in the exported PDF. If necessary, make these files available in your presentation via an external link. The files themselves should also be made to be accessible. You can find out how to insert a link in Insert Links.
Label embedded files and objects with a short text on the slide and assign an alt text. You can find out how to do this in Add Alt Text or Mark Object as Decorative.