If you created the PDF yourself then it would be obviously best to losslessly reduce the images before including them into the document (the PDF). The related muPDF is even available on the App Store. Ghostscript is available through a number of venues for macOS. Meaning: most of the time the savings gained might be very good, but sometimes the file size even increases.
Those given here should not perform any lossy conversions or strip away important document structures. Gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -dCompatibilityLevel=1.7 -dNOPAUSE -dQUIET -dPDFSETTINGS=/prepress -sOutputFile=Name_of_Outputfile.pdf To_Reduce_Input.pdf
Thankfully, a custom filter can be added. The problem is - the default filter used during conversion has very low conversion settings.