We can use the files concept to load the doc file content
into the textbox using the Multiline property.

we have many ways.This is one of the way.

using System.IO;


FileStream fstream = new
FileStream("Sample.doc",FileMode.Open,FileAccess.Read);
StreamReader sreader = new StreamReader(fstream);

txtFileContent.Text = sreader.ReadToEnd();


->change your Text Box Property into tghe multilien txtbox.