The ability to Compare 2 documents for revisions is a power feature of VisibleThread Docs that makes it easy to quickly identify changes between document revisions.
A small number of users have reported an issue when using this feature in that the excel spreadsheet produced by the process is empty.
What is actually happening here is that Excel file is being generated fine, however your local Excel installation is not opening it correctly.
This most likely caused by a recent Windows update that has impacted many applications that generate Excel files for download.
A list of possible workarounds can be found on the following thread from the Salesforce forum, which is also impacted by this issue.