Reference XObjects, PDF/X-5 and Acrobat 9
PDF 1.4 introduced Reference XObjects, but it has not been implemented in Acrobat in the intervening years. With the growing popularity of PDF/X-5g documents and interest in variable data printing (VDP), the Acrobat team decided to add support for Reference XObjects in 9.0.
Read more about this from Shradha, one of the Acrobat engineers who made it happen!
