In typical usage, "package leaflets" are printed materials that are enclosed inside the packaging of some consumer products, especially medical supplies. For medicine (a product that commonly comes in a package with a leaflet) the leaflet may provide dosage information, potential side effects, and other special instructions. This phrase seems like it might be a specialty term--it's not one that I hear often, but it is the proper name for brochures or pamphlets inside the packaging of any type of product. At the very least, it is probably not very common in American English.