You have to compress a PDF, be it to fit an arbitrary file size requirement, optimize for quick viewing, or save on data costs. There are several ways to go about doing so, depending on your platform, privacy needs, and how much time you have. On macOS, you don’t need to download any app. The in-built Preview app can reduce the file size of PDFs. For PDF files that have not been optimized yet, the Preview app in OS X can often reduce the file size considerably by passing it through an export filter. While it works great for text-heavy files, it’s not a perfect solution for every document out there, so you’ll want to run through the process with the document in question and compare it to the original to see if it helps.
How to Compress PDF Files and Reduce Size for free on macOS
Open a PDF in Preview – for most people just double-clicking on the PDF file should do just that.
Select File > Export.
Under the Quartz Filter option, choose Reduce File Size.