On Mac at least, any save to a PDF deletes the old file and writes a new one, rather than overwriting the old file. This means that metadata such as the Date Created is lost -- it's important for me to know when certain files were received, but then also to run OCR, add comments, or make other edits to the files. This breaks the standard Mac user interface and is only done by Acrobat (not Word, Preview, TextEdit, etc.)