Item Find increases the quality of the drops you get, not the quantity. This means that when loot drops for you, Item Find can increase the rarity of what you find. Example: You enter a dungeon and a common item drops. Before you get the item, your Item Find will kick in and you have a chance to receive a Rare or higher item instead of the common.
Item Find affects many things in the game: Equipment dropped in Zones, Raids, and Trials; Fusion Schematics, Coins, and Enchants.
Certain items are on different loot tables or do not have more than 1 rarity and are not affected by Item Find. This includes pet eggs and accessory boxes from the shop, zone cosmetics (they are unique drops), and crafting materials.