After upgrading to Sierra I can't change any document with Preview if it is saved under ~/Desktop. I get the error "The original document can't be chnaged, so a duplicate with your changes has been created." However, if I copy the same file to my Documents or Dropbox folder I can edit it. The folder permissions for ~/Desktop my Documents folder are the same:
drwx------+ 54 myname staff 1836 Oct 4 10:41 Desktop drwx------+ 26 myname staff 884 Oct 4 10:40 Documents
Permissions for the file is
-rw-r--r--@ 1 myname staff 247097 Aug 9 09:45 picture.jpg
Even if I change the picture and Desktop folder to 777 I can't edit it.