In an attempt to preserve one handed operation, (which was a human interface guideline priority for Apple), and still be able to placate the market with the larger displays that they craved, Apple introduced the ‘reachability’ function. By double tapping (not pressing) on the biometric scanner/Home Button, the upper portion of the UI is lowered so that the user can have access to the upper most app icon row while still only using one hand.
like I said….it depends on where where you place to cursor to edit. If you are editing in the upper third, the keyboard popup doesn’t affect the document. If you are trying to edit in the lower third, the keyboard will push the document up, sometimes to the top of the page, other times it would scroll to just above the keyboard.