Without these PDFs, we wouldn't know that Kunai are 20cm long, or that Paper Bombs contain a microscopic summoning seal.
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There are four primary databooks, often referred to by their Japanese titles: Book 1: Rin no Sho (Scroll of Facing) Without these PDFs, we wouldn't know that Kunai
These books confirm character ages, heights, blood types, and favorite foods. More importantly, they reveal facts never stated in the manga, such as: Without these PDFs
Here is an interesting report on key findings from the Naruto Databooks: