at the moment there are no direct APIs to achieve this.
You should retrieve the coordinates of the fields (with annotation.GetRect()) and put the text at that point.
It is not enough to read the text, you must also read the type of annotation (combo, checkbox, etc.) to put the correct text.
for now the library doesn't have this feature and the only way is to replace field values with some text using the functions of the premium license.
The flatten function will be implemented but we don't know the release times yet.