Where can I find the Open Pet Food Facts API ?

Open Pet Food Facts API is 98% the same as Open Food Facts API so if you use Open Food Facts API documentation you should have a pretty clear view of the Open Pet Food Facts API.

Note that features like the Knowledge Panels, Eco-Score, Nutri-Score are not supported (either due to the topic, or to the old codebase).

https://openfoodfacts.github.io/openfoodfacts-server/api/ref-v2/