It won't work on an iPad.
From Logitech's product support page:
K400 Plus keyboard works with both desktop and laptop computers and is compatible with the following operating systems.
Windows® 7 and later
Chrome OS™
Android™ 5.0.2 and later
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As you can see, it's not compatible with Apple's iOS devices.
http://www.logitech.com/en-gb/manuals/11916