An AI file won't open: five reasons, in order
Work down the list: the file is truncated, it was saved without PDF compatibility, it is not really an .ai, the app needs the network and has none, or the artwork is white on transparency and you are looking straight at it.
Pathside opens .ai, .eps, .svg and PDF on the phone →
1. The download stopped partway
The most common cause by a distance. A file that arrived over a patchy connection, or was sent through a service that truncated it, starts like artwork and ends nowhere. A viewer that checks for the end of the document will call this damaged. Ask for it again, or re-download it on a stable connection, and compare the file size with what the sender says it should be.
2. It was saved without PDF compatibility
Illustrator's Create PDF Compatible File option is on by default, and when someone turns it off the file contains only Illustrator's private data. Nothing but Illustrator can draw it. There is no fix on your side; ask for it again with the box ticked, or as a PDF.
3. It is not an .ai file
An extension is a label, not a fact. Renamed files are everywhere — a .cdr called .ai, a JPEG called .ai, a ZIP of assets called .ai. A viewer that decides from the first bytes rather than the name will tell you what you actually have.
4. The app needs a server and cannot reach one
If an app shows an error mentioning conversion, a failed request, or asks you to try again later, it was never opening the file locally: it uploads it and renders it remotely. On a weak connection it fails, and on any connection your artwork has left the device. An app that draws files on the phone has neither problem.
5. It opened fine and the artwork is white
White artwork on a transparent background, shown on a white page, is invisible. This is not a bug and there is nothing wrong with the file. A viewer that draws a transparency checkerboard shows it immediately; otherwise, try the artboard on a dark background, or export it as a PNG and look at it in a gallery.
Still stuck?
Two questions settle almost everything: how big should the file be (does yours match?), and can the sender open it themselves right now? If they cannot either, the file was broken before it reached you.
Pathside
An offline viewer for .ai, .pdf, .svg and .eps artwork. Files are drawn on your device and nothing you open is ever uploaded — it works the same in aeroplane mode. Viewing is free; exporting to PNG, JPG or PDF is a one-time unlock.