For Amazon I did it through KDP and uploaded a print quality version. And it turned out pretty nice for black and white! Everything came out sharp and clear, even when the pdf I uploaded was only in 300 dpi. I chose KDP initially because there is no upfront cost, and covered all that I needed for my comic.
IngramSpark is separate and that's what I'm looking at for my novel, since they distribute to libraries and bookstores and are not limited to only Amazon. I've heard good things about them! It does cost to upload to them ($49 for both print and e-book, a bit less if you're just going either or), but I think their distribution reach is worth it, as opposed to KDP which is restricted to Amazon.