To save synchronously to a Nevron Binary Text Document local file call the SaveToLocalFile method of the rich text view. This method doesn't work on sandboxed platforms like Blazor WebAssembly.

richTextView.SaveToLocalFile(@"D:\Documents\MyTextDocument.ntb");

To save a rich text document to a file asynchronously call the SaveToFileAsync method of the rich text view. Calling this method shows a loader over the rich text view until the saving completes. The method returns a promise, which you can use to get notified when the saving completes.

richTextView.SaveToFileAsync(@"D:\Documents\MyTextDocument.ntb").Then(
    delegate (bool success)
    {
    },
    delegate (Exception ex)
    {
    }
);

To save a rich text document to a stream call the SaveToStream method of the rich text view and pass the text format to save to:

richTextView.SaveToStream(stream, NTextFormat.NevronBinary);

To show the standard Save File dialog, use the SaveAsAsync method:

richTextView.SaveAsAsync();

If you want a specific format to be selected by default, pass it as an argument:

richTextView.SaveAsAsync(NTextFormat.NevronBinary);