To grab this info for PDFs, I believe the identity property loginName in
Acrobat JavaScript can help. The reference guide titled 'JavaScript for
Acrobat API Reference' says "The login name as registered by the
operating system." I cannot provide the exact steps as I have never
tried this.If this is your need, then I request some community member to
showcase an example of how to implement this in a PDF form generated
from Designer.