C# Requires Chilkat v11.7.0+

Replace Text in a Signed PDF (PreserveSignatures)

See more PDF Text Replacement Examples

Applying a text replacement rewrites the page content, which invalidates any digital signature on the PDF. While the PreserveSignatures property is true (the default), "apply" is refused on a signed PDF; "check" only reads and is always allowed. Setting PreserveSignatures to false allows the replacement; the signature field stays in the written PDF but no longer verifies, so the document should be signed again afterwards if a signature is wanted. C#

bool success = false;

//  This example requires the Chilkat API to have been previously unlocked.
//  See Global Unlock Sample for sample code.

//  The PDF used in this example is available at invoice_signed.pdf

Chilkat.Pdf pdf = new Chilkat.Pdf();
success = pdf.LoadFile("qa_data/pdf/invoice_signed.pdf");
if (success == false) {
    Debug.WriteLine(pdf.LastErrorText);
    return;
}

Debug.WriteLine("Signatures: " + Convert.ToString(pdf.NumSignatures));

Chilkat.JsonObject json = new Chilkat.JsonObject();
json.UpdateString("labels[0].label","Invoice Number:");
json.UpdateString("labels[0].value","INV-2026-0500");

//  "check" reads only, so it works on a signed PDF.
Chilkat.JsonObject jsonOut = new Chilkat.JsonObject();
success = pdf.TextReplace("check",json,jsonOut);
if (success == false) {
    Debug.WriteLine(pdf.LastErrorText);
    return;
}

Debug.WriteLine("check: ok = " + Convert.ToString(jsonOut.BoolOf("ok")));

//  "apply" is refused while PreserveSignatures is true (the default).
success = pdf.TextReplace("apply",json,jsonOut);
if (success == false) {
    Debug.WriteLine("apply refused: " + pdf.LastErrorText);
}

//  Allowing the signature to be broken:
pdf.PreserveSignatures = false;
success = pdf.TextReplace("apply",json,jsonOut);
if (success == false) {
    Debug.WriteLine(pdf.LastErrorText);
    return;
}

Debug.WriteLine("applied: " + Convert.ToString(jsonOut.BoolOf("applied")));

//  The written PDF is a full rewrite with the replacement.  The signature field is still in the document,
//  but the bytes it signed are gone, so it no longer verifies.
success = pdf.WriteFile("qa_output/invoice_reissued_broken_signature.pdf");
if (success == false) {
    Debug.WriteLine(pdf.LastErrorText);
    return;
}

Debug.WriteLine("Signatures after writing: " + Convert.ToString(pdf.NumSignatures));
Chilkat.JsonObject sigInfo = new Chilkat.JsonObject();
bool validated = pdf.VerifySignature(0,sigInfo);
Debug.WriteLine("Signature 0 verifies: " + Convert.ToString(validated));

//  Sample output:

//  Signatures: 1
//  check: ok = true
//  apply refused: ChilkatLog:
//    TextReplace:
//      DllDate: Nov  7 2026
//      ChilkatVersion: 11.7.0
//      UnlockPrefix: CHLKAT
//      UnlockStatus: 2
//      Architecture: Little Endian; 64-bit
//      Language: Visual C++ 11.0 / x64
//      VerboseLogging: 0
//      replaceTextOp:
//        op: apply
//        The PDF is signed: replacing text rewrites the content and invalidates the signatures (set PreserveSignatures = false to allow it).
//      --replaceTextOp
//      Failed.
//    --TextReplace
//  --ChilkatLog
//  
//  applied: true
//  Signatures after writing: 1
//  Signature 0 verifies: false