Comment by OneMorePerson

I'm trying to understand this cause it sounds fascinating but I don't get it. I don't have an advanced understanding of compression so that might be part of why.

If you compare an image to another image, you could guess by compression what is under the blocked part, that makes some sense to me conceptually, what I don't get is for the PDF specifically why does it compressing the black boxes I put have any risk? It's compressing the internal image which is just the black box part? Or are you saying the whole screenshot is an internal image?