I think what you are seeing as "white bars" are, in fact, the unprintable margins of the printer you have chosen as the default printer. When you export or print to a PDF, the print driver uses the known attributes of the printer to create the PDF. Most printers cannot print all the way to the edge of the paper, so the image is created with nothing to print in that outer edge, which then looks white in the PDF.
What you might do is to see if your printer can print all the way to the edge (Some photo printers can do that) and then select photo as the paper and select a size to use and see if that will create an edge-less print. Just remember to change it back so that the next "normal" print you do is done properly.
If your printer cannot print all the way to the edge, then I am out of ideas. I don't think you can install a printer on a Mac without having the printer attached to it or available to it, but if you can find out how to do that, then install a photo printer that can print edge-less photos and use that printer for your Keynote exports.
