Need to figure out best practices to get a smaller file when exporting to PDF

I’m looking for help and guidance with creating reduced file size PDFs from CorelDraw (Standard 2020). I am trying to publish an adult coloring book and I am limited to a 20MB file size. I have 55 designs that are all black and white line drawings. The Corel file is made up of importing these JPG designs into it, one per page. The Corel file is 100MB. When I export it to a PDF, the resulting PDF file is roughly 28MB. The file size reducer in Acrobat only can get it down to about 24MB. I don’t want to lose any of the line detail, so I’ve been trying to manipulate the JPG files to make this work. I cannot for the life of me figure out what is the best format for the JPG files to get the desired result. Low DPI and small dimensions? Low DPI and large dimensions? High DPI and large dimensions? High DPI and smaller dimensions? I started with 72 DPI files in Corel. This is the 100MB file. Then I replaced a bunch of the JPG files in Corel with 300 DPI files, and the Corel file went up to about 239MB, but the PDF export gave me roughly the same file sizes as before. Does anyone have any best practices for creating a smaller PDF file size with line art? I appreciate any suggestions.