test.razor

 [JSInvokable("ProcessDocument")]
 public void ProcessDocument(string document)
 {
     byte[] bDocument;

     // create a ServerTextControl instance to load the saved document
     using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
     {
         tx.Create();
         tx.Load(Convert.FromBase64String(document), TXTextControl.BinaryStreamType.InternalUnicodeFormat);

         // add additional text to the document
         tx.Selection.Text = "This document has been modified by .NET\r\n";

         // save back
         tx.Save(out bDocument, TXTextControl.BinaryStreamType.InternalUnicodeFormat);
     }

     // invoke the JS function 'loadDocument' to load back to the modified document
     _txtextcontrol.InvokeVoidAsync("loadDocument", Convert.ToBase64String(bDocument));
 }