Adobe Reader DOES NOT allow users to print double sided.  Therefore, it requires you to open the PDF file in Mac’s built in Preview.app.  

1) Press Control Key and click on PDF file that you want to print.

2) Select Open With and Choose Preview.app if it’s not set to default

3) Click on Preview dialog box and select Layout

4) Within the Two-Sided option, switch it from Off to Long-Edge binding