We have a signed PDF but signature is described without key "Sig".
As you can see in the standard PDF 32000-1:2008 - Table 252 - Entries in a signature dictionary, this key is optional :
"(Optional) The type of PDF object that this dictionary describes; if present, shall be Sig for a signature dictionary. "
But PDFBox seems to limit its research of signature only if this key "Sig" is present.
What is your position about that?