For reducing the file size of a PDF you can use PDF Optimizer in Acrobat Pro. Acrobat Preflight, Callas pdfToolbox or Enfocus PitStop can also perform this task.
There are also web services for shrinking PDFs. Apago is proving a free shrinking service at Apago tool by the same name. After uploading a PDF from the local computer or a Drobbox one you select one of five settings (High Quality, Website, E-Mail, Mobile and Scanned Documents).
After processing (which depending on the size of the PDF can take a while) you receive an email with a download link. On the download page you can see a nice graphic showing a comparison of the sizes of both PDFs:
If you login you can directly download the results or save in the Dropbox. But you are not seeing the graphic anymore.
Pdfshrink not only reduces the image resolution and compression but also removes other information like the PDF/X entries including the Output Intent. Depending on the ICC profile used this can reduce the file size by more than 1 MB. This feature is not available in the PDF Optimizer in Acrobat Pro!
However I have found a small shortcoming. The PDF is not optimized for fast web view when applying the setting Website.