You may need a Base64 encoder (losing compression) to make it preview-able in text editor, to save it then you need only to output it to a file
System.IO.File.WriteAllText(@"C:\Users\Public\TestFolder\WriteText.txt", str_your_base64_encoded_bit_array);
or directly (as it is a compressed file):
BitArray bit_array = . . . .
byte [] bytes = new byte[bit_array.Length / 8 + ( bit_array.Length % 8 == 0 ? 0 : 1 )];
bit_array.CopyTo( bytes, 0 );
File.WriteAllBytes( @"C:\MyFile.bin", bytes );