I was looking for a quick way to flip PDF files for the sake of adding e-signatures. I ran across a CLI utility called PDFTK.
sudo apt install pdftk
You can rotate using direction, north and south = 180 degrees, east, and west = 90 degrees. In my case, the documents were upside down so I flipped them 180 degrees south.
pdftk 'some pdf file.pdf' cat 1-endsouth output SomePDF.pdf