Preview Sierra Issues "original document can't be changed, so a duplicate with your changes has been created."

I there,

I've been having this issue with Preview after upgrading to OS Sierra:

Every time I try to modify anything to a PDF I receive this bubble message:

"The original document can't be changed, so a duplicate with your changes has been created."

And sure enough... a duplicate is created for every change. I don't want this, I want to keep the changes in the same single document.

Anyone has any clue how to fix this?

Thanks !