Help with exporting to PDF

I have a large map that I am trying to save as a PDF without the PDF file being to big.  My map is in color, but I am trying to save the file in B/W or Grayscale.  When I export to PDF, it file is 9,218 KB and I need it to approx. 3,284 KB.  I somehow did this last year but I can't, for the life of me, remember how to do this again.  I only use this software one time a year around Memorial Day so I forget how to do some of these operations.