Look at the PDF's properties to view the PDF's producer. Acrobat is not an "authoring application". The authoring application is may be Word, Publisher, InDesign, FrameMaker, QuarkXPress, Notepad, Office Libre or whatever.
Good to know is that PDF (the file format/technology that is defined by ISO 32000) does not tolerate the "editing" we are all used to when using a word processor application.
If you do not have the authoring file you could export the PDF content to a Word file. There'd be cleanup of course. So, cleanup - edit - and create a fresh PDF.
Of course you'd want to keep the new authoring file safe for any future editing.
Be well...