Finding EPUBs and PDFs of light novels at no cost isn't too tricky, but the landscape is a total mess. Half the sites promising 'free downloads' are just dumping grounds for MTL or bad fan-scans, and good luck getting a clean, official translation file. I usually start on places like J-Novel Club's site—they give you a massive free preview sample if you hook up a free account. It's not the whole thing, but it's high-quality. For older stuff, sometimes the fan-translators themselves host the EPUBs, but you gotta dig through their Discord channels or personal blogs. My bigger headache is the format war; some sites only have PDFs that are just scanned page images, totally unusable on a phone. EPUB is the real target, but you need to check if it's properly formatted or just a text dump with no chapter breaks.
Honestly, the most reliable 'free' method I've found isn't downloading at all. Scribd and some public library apps like Libby, if your library supports it, have a decent digital collection you can 'borrow' as EPUBs for offline reading. It feels more above-board than trawling through ad-infested aggregators. Still, for that specific series that's never been licensed, the hunt continues, and you end up with a folder of mismatched files. I wish more publishers would adopt a 'first volume free' model officially; it'd cut down on so much of this scavenging.