What could I use to hang a sign that is 20 pounds (hang from the ceiling)

Assuming this is wood-frame construction, you need to find a joist.

It’s like finding a stud when you want to hang something on a wall.

After that, a simple hook will do. 20 lbs is not a lot of weight if it’s hanging straight down.

Illustration

What @jaytkay said, with the provisos that:

1. There’s no wind load on the sign.
2. It’s not made of any fibrous material, unsealed wood, or has traps for water, and will be exposed to weather (rain, ice, snow, etc.) which will add to its weight.
3. It won’t be a type of “attractive nuisance” that will induce, say, children to play with it or hang from it, and nothing else will be hung from it.
4. You really intend to hang it “straight”. If you start adding lateral loads on the fastener in the joist – whatever that is – then your loading can increase enormously.

Is it the Krispie Creme sign? That looks heavier than 20 pounds.
I’d use a strand of wire on each side.

Two wires, two hooks, use your level. One wire will just let it blow in the wind. It will anyway, actually: 20 pounds is too light. Glue some
iron to the bottom edge…..

@Adirondackwannabe Yes, it’s the Krispie Kreme sign. The same one from a different seller. The seller claims that it’s twenty lbs. I’ll be hanging it from my kitchen ceiling.