If you have watermarked files in a data room, all files will be converted to a PDF for download. If you wish to have data room guests download MS Excel file in its native format, you can either disable watermarks for the data room or make an exception for Excel formats as follows:
Disable watermark for Excel files when you create the data room.
Alternatively, if you have an existing data room, you can:
Change the data room settings by selecting your data room > click on the more options icon, last icon from the left > Change Data Room Settings > Watermark > Disable watermark for Excel files.