Determining empty region in a PDF for barcode plac

Personally, since I deal regularly with PDF to bi-level bitmap conversion, I would simply render the PDF to a single-bit-per-pixel BMP and walk through the scan lines looking for runs of bytes containing 0xff values. But that is a messy solution, especially if you do not have the tools to hand, or do not have familiarity with twiddling bitmap bits at a low level. And it also does not help if your definition of 'empty' embraces a 'light gray' block as well as plain white, unless you have available a "nearest to black/white" dithering algorithm.  So it is practical for me, but might not be for you. But I would demand a high fee if someone asked me to code a tedious task like this anyway!