如何将外部OCR嵌入现有PDF？

I have a set of images over which I run an OCR application. This process results in a XML file with character offsets. Then I convert the images to PDF using Acrobat 9. Now, I would like to add the XML file information as an invisible text layer into the PDF in order to achieve a searchable PDF. Is there an easy and free way?I have a set of images over which I run an OCR