Comments by Cridbro (Top 3 by date)

I believe what you're actually looking for is to do a mail merging in C#, that is a task of importing data from some external source into your document. For example you create a table inside your document and place special placeholders (merge fields) into it and when merging the table will be filled with rows (for each record in your source) and cells (that will have values of those records). The article above demonstrates this by using a C# dll for word.

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".