I had the same issue with my portfolio and did a hell lot of research on net to compress it. So finally I figured this out and here it is - download ' Adobe Acrobat X Pro ' from adobe website (trial version is free). Open you pdf in that and go to File>SaveAs>OptimizedPDF here change your image resolution to '150 ppi for images above 200 ppi' and hit okay, that shoud work it reduced my 40 mb portfolio to 16 mb.
(PS:this resolution is fit for viewing on comp only not printing)