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. There are several free Windows software apps that will compress PDFs. One popular option is 4Dots Free PDF Compress, which supports everything from Windows 7, 8.1, and 10.
How to Compress PDF Files and Reduce Size for free on Windows