Macro warning combining PDF files

          We have a user who was able to combine PDF files using Acrobat DC but suddenly cannot do so. The warning says that the source of the file is untrusted and references AppData\Local\Microsoft\Windows\INetCache\Content.Word. That location sounds as though it ought to be untrusted. After that, a warning says that the "Security level is set to High. Please run the application which created this document, in the "Security Warning" dialog select the check box "Always trust macros from this source," and enable macros created by Adobe Systems, Inc."

          The stock advice I have been able to locate suggests running a repair on Office, but I have already done that to no avail. Following the advice in the second box doesn't seem to be possible as it looks as though I'd have to have a document created by Adobe Systems, Inc. to use to start the process. There is no way to add a publisher by name in the Options of Adobe or Word. Is there any other way to deal with this? My Internet searches have yielded very meager information on this apart from lots of people who said that repairing Office didn't help, and there doesn't seem to be anything else to try.