Just a quick comment about Affinity... they're currently offering a 90 day free trial (I think the usual is something like 10 or 14 days), plus 50% off the normal purchase price, so it's a good time to take a look at their software, if it appeals. https://affinity.serif.com

About your 2nd question, I should imagine any software that supports layers and blends (which is pretty much all serious image editors/painting software) would enable you to make a composite. I suspect your PDFs will be raster-based (i.e. pixels, rather than vector) since they're of historical maps, so something like Gimp (free) or Affinity Photo among others would do the trick. Without seeing the type of images you have, it's difficult to offer more advice. I've made composites from different photos in the past, mostly using the Pen tool to cut out the relevant parts, but I'm not sure how that will work with maps.