I don't know exactly what you are asking. If you are dealing with a fill-in pdf document attachment you save the attachment, open the saved file with your pdf program, fill it in, save it, attach the saved version to a return message.
FWIW, I won't return a filled-in form whose fields could be modified by the recipient unless absolutely necessary. My preference is to print the filled-in document as a pdf and return that.
I don't know exactly what you are asking. If you are dealing with a fill-in pdf document attachment you save the attachment, open the saved file with your pdf program, fill it in, save it, attach the saved version to a return message.
FWIW, I won't return a filled-in form whose fields could be modified by the recipient unless absolutely necessary. My preference is to print the filled-in document as a pdf and return that.