Save modified WordprocessingDocument to new file

If you use a MemoryStream you can save the changes to a new file like this:

byte[] byteArray = File.ReadAllBytes("c:\\data\\hello.docx");
using (MemoryStream stream = new MemoryStream())
{
    stream.Write(byteArray, 0, (int)byteArray.Length);
    using (WordprocessingDocument wordDoc = WordprocessingDocument.Open(stream, true))
    {
       // Do work here
    }
    // Save the file with the new name
    File.WriteAllBytes("C:\\data\\newFileName.docx", stream.ToArray()); 
}

In Open XML SDK 2.5:

    File.Copy(originalFilePath, modifiedFilePath);

    using (var wordprocessingDocument = WordprocessingDocument.Open(modifiedFilePath, isEditable: true))
    {
        // Do changes here...
    }

wordprocessingDocument.AutoSave is true by default so Close and Dispose will save changes. wordprocessingDocument.Close is not needed explicitly because the using block will call it.

This approach doesn't require entire file content to be loaded into memory like in accepted answer. It isn't a problem for small files, but in my case I have to process more docx files with embedded xlsx and pdf content at the same time so the memory usage would be quite high.

Simply copy the source file to the destination and make changes from there.

File.copy(source,destination);
using (WordprocessingDocument wordDoc = WordprocessingDocument.Open(destination, true))
    {
       \\Make changes to the document and save it.
       wordDoc.MainDocumentPart.Document.Save();
       wordDoc.Close();
    }

Hope this works.

This approach allows you to buffer the "template" file without batching the whole thing into a byte[], perhaps allowing it to be less resource intensive.

var templatePath = @"c:\data\hello.docx";
var documentPath = @"c:\data\newFilename.docx";

using (var template = File.OpenRead(templatePath))
using (var documentStream = File.Open(documentPath, FileMode.OpenOrCreate))
{
    template.CopyTo(documentStream);

    using (var document = WordprocessingDocument.Open(documentStream, true))
    {
        //do your work here

        document.MainDocumentPart.Document.Save();
    }
}