Leonie ... I might have figured it out!! Don't have time to play around with it now, but try this out. Open a large PDF file that you want to shrink in size. Go to View -> Tools -> Document Processing. There should be a toolbar that opens up on the right hand side of the screen beside the document. One of the selections will be to Optimize PDF. When I did that, I had a 12 MB document shrink down to less than 1 MB!! You can play around with the slider to choose a smaller / lower quality vs a bigger / better quality. I don't know why they hid this feature there, but I think that is what we are looking for. I highlighted the command and dragged it up to my main toolbar so that it is easier to access next time.
Hope this helps!! Let me know how it works for you.