I'm making a tri-fold brochure that will fit on a 8.5 x 11 piece of paper. I would like it to completely fill the paper, however it keeps printing with white borders around it. How can I fix this?
At first check, if the printer driver or the printing dialog allows removing the unprinted margins. Obviously it doesn't because you are here.
Get a printer which can print the wanted area. Many printers leave a white border at the edges.There's a good reason: Spraying ink into mechanism or paper path creates a mess.
Or reduce the wanted image size and slice the borders off mechanically after the print has dried.
I have printed sometimes a big area in pieces. The slicing isn't easy, but it's possible with a sharp straight line making quided razor blade type cutter.
If your printer happens to accept bigger papers, use them and scale your image a little bigger than the wanted area to get some tolerance for the mechanical slicing.
That oversize has a pro term = bleed area. Printing houses demand it.