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There are a number of options. Do you know what compression the existing PDF is using? If it’s already using JPEG 2000 (JPX), it’s unlikely you’ll be able to compress it down much further. You can use k2pdfopt with the -i option to see this (or from the GUI, the Help menu -> PDF File Info).
You want to start by using “-mode copy” as your conversion mode from k2pdfopt (from gui—it can be selected in the little “Conversion Mode” drop down—top right). Then there are various options to reduce the file size:
-cmax <value> sets the max contrast. Getting the contrast up should make it so you can use fewer colors.
-bpc <value> sets the number of bits per pixel. Default is 4.
-jpg <quality> will store the page as jpeg images with adjustable quality. Otherwise the pages are stored essentially as .png which is lossless.
-g <value> sets the gamma value and defaults to 0.5. This is done to darken the text. Set to 1 if you don’t want to change this.
-odpi <value> will set the output dpi. Lowering this will reduce file size. Default=150.
-c- will turn off color output (grayscale) which will save on size
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