Exporting in a High Resolution Format

It all depends...

What the pixel size of your keynote?
How is it going to be printed? Ink jet or a print shop.
If it's going to be printed by a printing shop 150 dpi is low rez.

Since you work a print shop, you know Photoshop.

In Photoshop you can change the dpi (no resampling). As long as you have enough pixels to work with...

Your keynote needs to be 2250x1913 (there or abouts) to produce an 8.5x11 at 300 dpi.

Make the keynote 2250x3300 (for a 300 dpi 8.5x11 on paper). Import the keynote (tiff) into Photoshop. Open image size under the image menu. If the keynote is at 72dpi, unclick; scale style and resample image. Set the dpi to 300. The will be no resampling of the image because you have enough pixels to work with.

now if you don't have enough pixels to work with...there's really nothing you can do.

I worked in pre-press for 28 years. People would send in 72 dpi images. Luckily if they set the pixel size high enough we could sometimes get 150 dpi. Which at best is 85 lines. if not we had to asked for rescans..