Guilherme Rambo said is correct, not the Lewis42.
Suppose a pdf file('a.pdf') in Asserts.car.
You will get three png files(a.png/a@2x.png/a@3x.png) if using cartool, file format had been changed.
But, if you using AssetCatalogTinkerer, you can preview the a.pdf file(no file format changed).