Read Mainframe text file using streamreader

Hi All,
I have a text file sent from mainframe. I read that file using the StreamReader.Like
if (File.Exists(filepath))
{
                StreamReader sr = new StreamReader(filepath);
                StringBuilder buffer = new StringBuilder();
                buffer.Append(sr.ReadToEnd());
                sr.Close();
}

Case 1:After i get the text file from mainframe and try to read it directly.
when i debug and see the value in the buffer, it does not show me all the contents till the end.
Case 2:After i get the text file from mainframe, I open the file and save it manually again with the same file name.
when i de ...