Procreate, no install
Seal the file.
Procreate keeps its own record inside every file: the time-lapse, the tracked drawing time, the layers. Drop the file here and DrawDog seals that record with a signed, timestamped certificate before anyone could edit it. The file itself never leaves your device.
What this is, honestly
Sealed, not witnessed.
The desktop app watches work being made: which app is in front, the pen moving, the file growing. Nothing can do that on an iPad; iPadOS does not let one app see another. A sealed record is the next best thing. It takes Procreate’s own account of the file and locks it at a moment in time, so it cannot be edited afterwards and a client can check it matches the file they were sent.
It cannot catch tracing over an imported image, and a Procreate document can be edited before it is sealed. The certificate says both. It is labelled Sealed record, never Certified human-made. The full method.