Well, if the DigestValue isn't changing, then it definitely isn't doing what you want.
My best guess is that the information you want is embedded in the value of the V Contents. This value is about 17 kB in size, and looks like it contains a lot of interesting stuff. However, I think it may be a fair amount of work to decode this, and I don't know if it is worth the effort. I have looked at the digital signature part of the PDF documentation, and it references some RFC's that are very hard to follow. - Phil