I can produce such annotations when adding annotations to a PDF
attachement from the standard mail app on my iPhone (iOS 12.1).
They currently all show as "ÿþÚ" rather than the actual string content.

UTF16-BE vs UTF16-LE is detected by inferring the endianness from the
first two bytes of the string (0xFF 0xFE and 0xFE 0xFF aka Byte Order
Marker).