If you are experiencing issues opening XLS files, we are aware of an issue with Microsoft Office. Microsoft Office in a update in 2016 changed settings during an update.
You can view the original details here: https://answers.microsoft.com/en-us/office/forum/office_2010-excel/excel-2010-kb3115322-opening-downloaded-files/19b8ab42-d0c4-402d-aa79-ad86e191fcc9

How to resolve the issue:

1. Open Excel.
2. Go to Options --> Trust Center --> Trust Center Settings --> Protected View
3. Uncheck "Enable Protected View for files originating from the Internet" check box.
4. Close Excel
5. Try to reopen the file.

If you are still unable to open the file, try a different version of Excel or another spreadsheet program like Numbers (for Apple) or OpenOffice.