Duplex default in Acrobat X My MacOS and Acrobat Pro version are as follows.
Mac OS X 10.7.5
Acrobat 10.1.6

When I'm printing PDF documents, I can never set 'duplex printing' as a default setting.
I think I could do it in previous versions.
What should I do?
Try upgrading to Acrobat XI if you can. I've found it's so much better than X in so many ways that it was totally worth the price. I'm not at my Mac now, but you should be able to set the profile you want in either the Print or Print Setup dialog, then save it as a default.