To preprocess for camera scan, How to undistort photo of unfolded paper out of mail envelope? Hardcopy is lost and replacement is a month away.

Keynote is not an image manipulation application.

There are apps out there that have image editing capabilities, including distortion and perspective control. Changes are one of them would enable you to straighten the image and recapture the QR code.

Photoshop ($$) and Gimp come to mind. Not sure of their iPad compatibility, though.

Once you straighten the image, the built-in photos app will decode the URL.

SunnyInToronto wrote:

1. To preprocess for camera scan, How to undistort photo of paper unfolded from mail envelope? The hardcopy is lost and replacement is a month away.

You may need a new original and undistorted version of the QR code image.

The QR code is positioned where paper is folded; without hardware accessory to remove following image distortions

crease from folded paper

2. Hexagonal 

Can Keynotes remove distortions from following image?

Not a Keynote functionality to undistort an image.

https://discussions.apple.com/content/attachment/b047208c-b109-43c7-a961-d72fe57f2d75

I used magnetic bulletin board and to flatten document area to scan, on two opposite corners of block of text, I placed left and right magnets that come with Bluetooth headset (turn on/off headset). 

For me, This is more practical than carrying another jig specialized to flatten documents.