If you have saved a Microsoft file to your downloads folder in your Chromebook's storage, you may find that the file will not open. However, you are able to open a similiar file (or even the same file) from your Google Drive
You will need to add the following extension:
After adding this extension, Microsoft files will open from your Chromebook's storage in your browser in Office Compatibility Mode. You can then save them as a Google file (docs, sheets, slides) in your Drive.