Settings - Optimize PDFs
ShortPixel Image Optimizer can also optimize PDF files and the associated thumbnail files generated by WordPress. When you enable this option and you also have the " Optimize media on upload" option enabled, the PDF files will be optimized automatically right after being uploaded, as well as through Bulk Processing. In case you use high-definition PDFs or detailed drawings, we recommend that you first test the optimization manually before enabling this option, because it may result in quality loss.
When this option is not enabled, the PDF files can still be manually optimized by clicking on the Optimize Now button in the Media Library list view, next to each file under the ShortPixel Compression column.