The signing feature in ODF/OOXML is implemented by working directly on the ZIP storage in xmlsecurity/. This means that in the PDF case it’s necessary to work on the PDF file directly, except that we had no such PDF tokenizer ready to be used.
Code under xmlsecurity/source/pdfio/ now is such a tokenizer that can extract info from PDF files and can also add incremental updates at the end of the file, this way we can make sure adding a signature to a file won’t loose existing content in the file. This is fundamentally different form the usual load-edit-save workflow, when we convert the file into a document model, and work on that.