It is possible to compress a PDF in Preview. I have done it many times and the results vary in terms of savings and quality.
*In the Preview app on your Mac, open the PDF, then choose File > Export.
*Click the Quartz Filter pop-up menu, then choose Reduce File Size.
Remember that a scanned PDF file is an image just as a JPEG is an image. Image compression algorithms do their thing by discarding data. The variation in results are a function of the initial scan resolution (how much data you start with) and the compression level (how much of the original data is discarded in the compression process).