Brute Force Attack in excel

You could use an excel component for .NET, it is able to read your excel files with C# in memory. Even the protected (encrypted) ones, however for them you need to provide password (see here).
So what you could do is use a brute force iteration for password combinations and for each use something like the following:

string password = "ABC";
bool success;

try
{
var ef = ExcelFile.Load("Sample.xlsx", new XlsxLoadOptions() { Password = password });
success = true;
}
catch(FileFormatException)
{
success = false;
}

You do this until "success" is "true".