I had the same problem and I finally managed to solve it using CleanMyPhone (Mac application)
If your problem is like mine, you have an incomplete Keynote stucked on your iPad.
The only way to download it again and successfully install it, is to remove the stucked Keynote from your device first.
I was unable to delete from the iPad using the standard way to delete an app from an iPad (hold the icon and tap the X)
So if you have a Mac, install CleanMyPhone and connect your iPad to your mac with USB cable.
Then look for Keynote icon under Applications in CleanMyPhone. I say icon because the title "Keynote" was not showing, there was only .app beside the Keynote icon.
Mine was stucked at 23megs. Keynote is 463megs.
Once the stucked Keynote gone, you will be able to install it.