Question:
I want to sign a PDF with multiple fields using the same SignatureHandler instance. When I do this and open the signed PDF, I am getting invalid signature errors. How can I sign a PDF with multiple fields using the same SignatureHandler?
Answer:
Please be aware that most other PDF readers out there does not allow for having more than one field signed at the same time (in one save process). PDFNet allows for this process and adds as many skips as possible in byte ranges of data to sign as per the PDF specifications (see the PDF reference for details). Most PDF readers, however, does not validate a signature with byte ranges like this. If you wish to sign multiple fields in a document using PDFNet which will validate in most PDF readers, you will need to save the PDF document each time you call the method Field.useSignatureHandler. Then after that, you will need to re-open the document, repeat the signing process, then save again. It does not matter whether you wish to use the same SignatureHandler instance or not for all the signature fields you wish to sign. For example:
PDFDoc doc = new PDFDoc("/path/to/doc.pdf");
sigHandlerId = doc.addSignatureHandler(mySigHandler);
Field sigField = doc.getField(“Signature1”);
sigField.useSignatureHandler(sigHandlerId);
doc.save("/path/to/signed_doc1.pdf", SDFDoc.e_incremental, null);// Re-open the doc
doc = new PDFDoc("/path/to/signed_doc.pdf");
// Re-add the signature handler…
sigHandlerId = doc.addSignatureHandler(mySigHandler); // we can re-use the existing SignatureHandler instance
sigField = doc.getField(“Signature2”);
sigField.useSignatureHandler(sigHandlerId);
doc.save("/path/to/signed_doc2.pdf", SDFDoc.e_incremental, null);