Open the PDF in GIMP and find the layer for the map (you can just open a small subset of the pages to make things easier to find). Then just make a selection and copy that into a new image and then export.
Doing it this way converts the PDF to an image with each page being on its own layer, so you hide the top most layer/page when you are done with it and move your way down the stack. This is how I'm doing it for my game of Horror on the Orient Express. And actually for anything where I'm entering the info from PDF.