Hunting for a mobile-friendly doujin site? I've tried a bunch and there are a few that feel slick on phones without making me fight the layout. For straight-up browsing and buying, BOOTH (the marketplace tied to Pixiv creators) is my go-to — their storefronts are responsive, images scale nicely, and downloads usually come as ZIPs or
pdfs that my phone handles fine. Pixiv itself also works well in its app and mobile web view for discovering artists and links to their shops. DLsite is another strong option if you're into doujin games and indie comics; the mobile store and reader are fairly stable and oriented toward purchases and downloads rather than clunky desktop layouts jammed into a small screen.
If you want creator-support style platforms, Fantia has a mobile-friendly interface for subscription content and creators often post optimized viewing formats for phones. For legacy Japanese shops like Melonbooks and Toranoana, the mobile experience varies — Toranoana's mobile pages have improved, but sometimes you'll end up redirected to simpler listing pages and need to switch to their reader or download chapter files. For adult material, Fakku is one of the few licensed English platforms with a solid mobile reader, if that's relevant to you.
A couple of practical tips: use a browser with a decent built-in reader or a long-image viewer for page-by-page scrolling, save purchases to a cloud drive so your phone can stream them, and prefer stores that sell official digital files to directly support creators. I love the freedom of flipping through doujin on my commute — makes the commute feel like a mini con, honestly.