Lua 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. Lua

local chilkat = require("chilkat")

-- 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

local pdf = chilkat.Pdf.new()
local success = pdf:load_file("qa_data/pdf/invoice_signed.pdf")
if not success then
    print(pdf.last_error_text)
    return
end

print("Signatures: " .. pdf.num_signatures)

local json = chilkat.JsonObject.new()
json:update_string("labels[0].label", "Invoice Number:")
json:update_string("labels[0].value", "INV-2026-0500")

-- "check" reads only, so it works on a signed PDF.
local jsonOut = chilkat.JsonObject.new()
success = pdf:text_replace("check", json, jsonOut)
if not success then
    print(pdf.last_error_text)
    return
end

print("check: ok = " .. tostring(jsonOut:bool_of("ok")))

-- "apply" is refused while PreserveSignatures is true (the default).
success = pdf:text_replace("apply", json, jsonOut)
if not success then
    print("apply refused: " .. pdf.last_error_text)
end

-- Allowing the signature to be broken:
pdf.preserve_signatures = false
success = pdf:text_replace("apply", json, jsonOut)
if not success then
    print(pdf.last_error_text)
    return
end

print("applied: " .. tostring(jsonOut:bool_of("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:write_file("qa_output/invoice_reissued_broken_signature.pdf")
if not success then
    print(pdf.last_error_text)
    return
end

print("Signatures after writing: " .. pdf.num_signatures)
local sigInfo = chilkat.JsonObject.new()
local validated = pdf:verify_signature(0, sigInfo)
print("Signature 0 verifies: " .. 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