Comment by jaffa2

Thanks for the message. Im talking about MRC ( https://en.m.wikipedia.org/wiki/Mixed_raster_content ) not just invisible text layer over image.

You can achieve colour PDFs smaller than group4 binary compression of the same images. And 10x smaller than a Jpeg compressed PDF

I handle many scanned documents so my source data is typically a 300ppi image of a book/document/newspaper etc