DocX document = DocX.Load(@"F:\BIIT\Project\LetterTemplate\sample.docx");
//Insert a Text into the document.
Paragraph p3 = document.InsertParagraph();
p3.Append("to introduce ").Italic().Font(new FontFamily("Times New Roman"));
p3.Append(" S/o ").Italic();
p3.Append("of our institute was enrolled in");
p3.Append("program under University registration No. ");
p3.Append("during academics session ");
p3.AppendLine("In this connection I would request you to provide him the opportunity to acquire practical knowledge and would appreciate your willingness to accept him as");
p3.AppendLine("in your esteemed organization");
As I looked in the code, you can't make that if you use Load and Save, because you are opening the document and append text.
So you have the option to create a new document, and append the text, or also you can use SaveAs as saving method, and make a differente filename for each document you create.