Depends what you mean by 'download' and what you're trying to do with it. The official English publisher, Yen Press, sells physical copies and ebooks through stores like Amazon Kindle, Kobo, Google Play Books, and Apple Books. Those you can definitely download for offline reading through their respective apps. The digital files are usually DRM-protected EPUBs or something similar, so you're tied to that app's ecosystem, but it's a clean, legal copy.
If you're asking about fan-translated PDFs or EPUBs floating around on aggregator sites, yeah, those exist too. You can technically grab them, but the quality is super hit-or-miss—bad formatting, machine translation slop past certain volumes, missing illustrations. I got a few that way years back before the official translations caught up, and it was a mess. Honestly, for a series that's still ongoing in English, the official route is less headache, even if you have to pay per volume. The fan scans are really only worth it for the later volumes that haven't been licensed yet, and even then, you're rolling the dice.