The 5.1.2 version of PDFOptim for Mac was available to download on the developer's website when we last checked. We cannot confirm if there is a free download of this app available.
Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. The program lies within Design & Photo Tools, more precisely Viewers & Editors. This software for Mac OS X is an intellectual property of RootRise Technologies Pvt. Ltd.
From the developer:
PDFOptim is a simple and intuitive way to compress, reduce, and optimize PDF file size. It provides easy and effective ways to compress large, image-heavy PDF files using an extensive set of options and presets. Supports batch processing and provides real-time original as well as optimized PDF document preview for side-by-side quality comparison.
You may want to check out more software for Mac, such as jPDFOptimizer, which might be similar to PDFOptim.