Does Pokemon Go attack power change?

A move's Power rating is fixed, so for example, Aerial Ace has the same Power no matter which Pokemon uses it. However, actual damage depends on several variables including level. A higher level Pidgeot will generally deal more damage.

The damage formula has been empirically determined. GamePress has a detailed article on Damage Mechanics, and the formula looks like this:

Damage = Floor(0.5 * Power * (Atk / Def) * Multipliers) + 1

Multipliers consists of several possible factors such as the Pokemon's level.

For sure, the number never changes

Other than that, there's still a lot of speculation going around, but for what I saw most recently on reddit and my experience, the damage dealt by an attack only depends on the attack power and the CP of the defending pokemon. It doesn't seem to depend on the attacking pokemon CP.

Edit : mons indeed have an attack stat, probably used somewhere.

To the history board:
In all Pokémon games there was an attack value (as well as an accuracy value) for each attack. This number never changes.

Does this mean the attack always deals the same damage? No!
The attack value is but one part of the calculation.

Another part of that calculation is the attack power of the Pokémon, which indeed grows as you level up.
This stat is rumoured to exist in Pokémon Go as well.

Will it always just deal out just 25 damage despite how strong the pokemon is?

No, but the base damage value of Aerial Ace will always be 25