How to convert richtextbox output into bytearray?

The RichtextBox[^] has a Rtf property holding text and format directives. It is a string property.
You could convert it to byte array this way:

string rtf = myRichtextBox.Rtf;
byte[] bytes = System.Text.Encoding.UTF8.GetBytes(rtf);

Here I used UTF8 encoding, but that choice is up to you.

Hope this helps.

EDIT: Later, if you want to recover original string from byte array, it can be done this way:

string originalRtf = System.Text.Encoding.UTF8.GetString(bytes);

You have to use the same Encoding in both operations.