I received a signed PDF document requiring two authorizing signatures which does not allow modification (checked in File:Preferences). I created a self signed digital ID with Acrobat DC and set the signature as the default. When the document is opened in Sign & Fill the only option available is to validate the existing digital signature. All other signing options are grayed out. I have tried saving a copy of the file using File:Save As, but I could not see any option with that method control further modification. How does one add another digital signature to the file?
If you sign a document using "Fill and Sign" feature, it will flatten the whole document.
So you should try digital signature option under tools rather than "Fill and Sign" feature to sign the document.
And if the document is not allowing you to sign the document it is most likely the first signer might have locked the document after he/She signed it.
Let us know if that helps you any.
Why is this so difficult?
There is no "digital signature" option under tools.
Look at "Certificates" under tools.