'utf-8' codec can't decode the file content

I want to read this file content:

import controller.MainController;

/**
 *
 * @author Terkel
 */

    /** Creates new form InfoFrame2 */

so I use

with io.BytesIO(targetfile.data_stream.read()) as f:
    data = (f.read()).decode('utf-8')

However, the results show only the first line and could not read next line, which is /**, and gives me this error:

'utf-8' codec can't decode byte 0xe6 in position 5960: invalid continuation byte