char[] buffer = new char[104857600];
string text = "";
FileStream fstream = new FileStream("document.txt", FileMode.Open, FileAccess.Read);
using (var sr = new StreamReader(fstream))
{
    int bytesRead = 0;
    while ((bytesRead = sr.Read(buffer, 0, buffer.Length)) > 0)
    {
        text = new string(buffer);
        // обработка текста
    }
}