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