Document will open in compatibility mode, won't automatically save to the cloud etc.
Can be fixed by changing the file type from a xls or doc to a xlsx or docx
It will not function properly without the x.
Have also noticed particular document types like csv files won't save formatting changes and again have to be made a xlsx to full benefit from sharepoint features.
Just do a save as and change the file type. Delete the old file type once done.
