May be there are easier ways to print PDF files to exact 50% scale. Adobe Reader does not have this print to scale feature. We tried using the scaling feature built in with the printer. But this resulted in non standard size sheets, which we didn't want.
We found a very easier method. Just open the PDF using Autodesk Design Review 2010 (free software) and this software has amazing capability to print to any size / scale. Just wonderful!