get original content of a pdf signed with itextsharp
I'm trying to get the original document of a signed PDF in order to compare it's hash with an stored doc.
This is really easy when the document has several signatures, with acrobat reader you can go the previous revision of the document save it and that's it.
Surprisingly this does not work with the first signature, where there is no straight forward way to get the original data.
As it is not possible to do it with the reader I have tried programatically with iTextSharp. However although I have googled deeply I have not found how to do it. The only relevant post I found is this one but no solution is offered.
Has anyone faced this problem and found a solution?