Does reading through ibooks or Kindle for iPad use data?

I am looking to buy an iPad mini or iPad and want to use it for apps for our preschooler, reading, and surfing the web.

If you are reading using either iBooks or the Kindle app - is that considered to be using data? Or is just using stored content on the device? I am trying to figure out how much GB I would need to get per month through our carrier.

iPad

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