Issue with Excel - "Excel file format is not valid"

Hey Tombromgrove,

Just to add some additional information to what Mark said. 

The issue you seem to be having is probably be due to a limitation of the applications that you're using to open the files in question.

This means that there are files that your applications can't process correctly since they have file paths that are too long ( the file path from your screenshot is a great example). 

This behavior is present if there are more than 255 characters in the entire file path. This means from the C:\ all the way to the extension of a file. Note that  the latest Dropbox desktop application versions do support for Dropbox to read and write to locations that are longer than 255 characters and files will sync to and from those locations, however the applications used to open said files (e.g. Word, Notepad, Excel, etc.) might not support reading from these locations with long file paths and may show error messages like the one you get when trying to open, save or edit these files. This is out of our control since the application developers would have to add support for long paths to resolve the issue.

You should be able to resolve the issues with your applications processing these files by simply renaming the file to a shorter name, renaming a folder to a shorter name, or reducing the number of sub-folders. 

I hope this helps!