If the author value is present, you'll find it in the
Please consult ISO-32000-1 table 170 "Additional entries specific to markup annotations".
It defines a key named
/T that is described as an optional text string that
shall be displayed in the title bar of the annotation's pop-up window when open and active. This entry shall identify the user who added the annotation.
The value you seek is not available for every type of annotations, only for markup annotations.
So when you have this in Adobe Acrobat:
You'll find this inside the PDF:
You already have the
/Type entry, now you should also look for the
If it is missing, the author of the annotation can not be retrieved.