I could probably automate it with Automator on my Mac. I'll look into it. I just want to shave a few 100 pixels off.
I just opened one up, and it's 1988x3056, the iPad Mini is 1024x768, and my computer monitor is 1366x768 in comparison.
This is how it looks like at 100% on my monitor:
I have to shrink it down to 25% before it fits on my screen.
At 828x1273 it's only 432KB, compared to the original 1.4MB.
This shouldn't be a problem I don't think? The text and graphics look fine at this size. I would keep my originals of course, and just have these smaller versions for the iPad.