Shrink all images in pages document
Preview can reduce the file size though you don't have much control over exactly what it does and how. For that you may need a purpose-built PDF editing tool, or Adobe Acrobat Standard or Pro.

Open the file in preview and then go FILE > DUPLICATE. A new identical window will open. Now save the new file and from the Quartz Filter menu select CompressPDF. Give your document a different name or location and click save.

Depending on what is going on in your PDF you should get a smaller file. But again you will have better control over the process with one of Adobe's Acrobat products or one of the third party PDF tools available for the Mac.