How to Open a .puz File on iPhone
Someone sends you a crossword — AirDropped, emailed, downloaded from a
constructor's site — and it lands on your iPhone as a file ending in
.puz. You tap it. Nothing you'd call "opening a puzzle" happens.
Mail shows a generic file icon. Files might offer no app to open it with at
all. It isn't broken. It's just a format iOS was never taught to read.
Why iOS doesn't know what to do with it
.puz is a small, old binary format that predates the App Store
by well over a decade — it was built for Across Lite, a desktop
crossword player, and it stuck around because it's simple enough that
constructors, puzzle syndicates, and crossword software have kept using it
ever since. iOS has no built-in reader for it, the way it has one for PDFs
or images. Mail, Files, and Preview just see an unrecognized file extension
and shrug.
What actually makes it work
Opening a .puz file on iPhone requires an app that has
specifically registered itself as a handler for that file type. That's a
real, deliberate step a developer has to take — it's what makes
"Open In…" show an actual crossword app in the list instead of
nothing. An app can be a perfectly good crossword solver and still not do
this, if importing other people's puzzle files wasn't part of what it was
built for.
A note on .ipuz
You'll also run into .ipuz files, a newer, JSON-based format
designed to handle puzzle types .puz can't — cryptics,
variety grids, that kind of thing. Same story applies: it only opens if the
app on your phone explicitly knows how to read it.
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To be clear about what Ultimate Crossword
actually solves here: it won't make a .puz file tap-to-open
from Mail on its own — that would need the OS-level registration this
piece just walked through, and that's not something currently planned.
What it does do is let you import a file you already have directly into
the app yourself, solved fully offline, alongside the free daily puzzles
and a built-in generator. It hasn't shipped yet, but importing your own
puzzle files is a first-class feature, not an afterthought.