How to import custom trees

I would like to use some custom trees in Unity. Is there some guidance which format is the best/easiest to import into Unity and are there some tutorials about the best way to do this? Also, are there some guidance what is the max limit of polygons. I noticed that there is actually an Asset (Custom Tree Importer) but it at least seems that there are some limitations what kind of trees it can import.

Foe example, I would like to import this tree to Unity. https://www.silva3d.com/?product=forest-trees-pines-bundle1