It seems that the way Apple is handling annotations in their built in tool has a nasty side effect - if you open (without modifying) a PDF that has handwritten annotations in it, the file modification date is updated.
Interestingly enough, this does not happen with other kinds of annotations (such as highlights). Furthermore, this is not a bug in Files.app, but a system wide problem. Opening a PDF with handwritten annotations that is attached to a note in Notes.app will update the note modification date as well.
Interestingly enough, this does not happen with other kinds of annotations (such as highlights). Furthermore, this is not a bug in Files.app, but a system wide problem. Opening a PDF with handwritten annotations that is attached to a note in Notes.app will update the note modification date as well.