As far as I know, there are many ways to compress a PDF file on Mac, such as strip out unwanted objects, remove tags and reduce the size of images. Hope following methods can do you a favor:
1. Full-featured Software- Cisdem PDFCompressor for Mac (https://www.cisdem.com/pdf-compressor-mac.html)
2. Mac Built-in Tool- Preview
3. Online Compressor- Smallpdf.com