Downsampling while retaining image masks

Acrobat X ( mac) I have a print pdf which ids 67.8 mb's I need to get the file underneath 50mb so I can upload the pdf to Amazon. Problem is I have to resample the pdf and when I do I get the message that image masks wer not converted. Without those masks the file looses the colored background.

I'm a little confused on how I can do this correctly.

Any help?

RD