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Choose preservation or reconstruction
These are different workflows with different evidentiary value. If you have the original PDF, preserve those exact bytes; if you do not, recreate a document from available data without presenting it as the historical original.
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| Workflow | When to use it | What it can establish | What to retain |
|---|---|---|---|
| Preserve the original | The issued PDF is available. | A digest comparison can show whether the file being examined matches bytes recorded earlier. | The unaltered PDF, its original filename, its source and receipt time, and its recorded digest. |
| Reconstruct from data | The original PDF is unavailable and a new rendering is needed. | That a new PDF was generated from identified data and rendering inputs; not, without byte-level comparison, that it is the original. | The invoice data, template revision, renderer and dependency versions, fonts, locale, timezone, configuration, and generation details. |
Preserve the original PDF and verify copies
Keep the received artifact intact
Save the original in a controlled evidence location. Do not edit or re-save it during intake. Record the filename, where it came from, and when it was obtained so the file has a basic provenance trail.
Calculate and record a SHA-256 digest
Node.js Crypto provides createHash('sha256'), followed by hash.update(data) and hash.digest(...), for calculating a digest from file bytes. For large files, the Hash API can be used with a stream. Record the algorithm and calculation time alongside the artifact. See the Node.js Crypto documentation.
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When you copy or transfer the PDF, calculate its digest again and compare it with the recorded value. A match means the bytes examined match the bytes represented by that earlier digest. A mismatch means the bytes differ and should be investigated; do not silently overwrite the original recorded digest.
A digest is not proof that the invoice data is true, that a particular person authored the file, or that a court will accept it. Retain the relevant source records and provenance information as well.
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Reconstruct an invoice when the original is missing
Retain the inputs and rendering environment
A new PDF depends on more than the invoice fields. Preserve the source data, template revision, renderer and dependency versions, fonts, locale, timezone, and relevant configuration. Changes in any of these may affect the rendered document, so do not assume that identical business data will produce identical PDF bytes.
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Label and document the new file
Keep the newly generated PDF separate from the original evidence location and label it as a reconstruction. Record when it was generated, which inputs and versions were used, and why the original was unavailable. Do not describe it as identical unless a byte-level comparison with the original supports that claim.
Validate PDF signatures separately
A digital signature applies to specified byte ranges of a PDF, not simply to the visible page content. ISO 32000-1 describes validating a signature by recomputing and comparing the digest over its specified byte range. It also describes incremental updates that can preserve an earlier signature’s byte range and allow the signed document state to be recreated when that signature validates. Those properties do not mean arbitrary edits—or a newly rendered PDF—preserve a prior signature. See the ISO 32000-1 specification.
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If a PDF is signed, validate the signature and record any later revisions or changes rather than treating the file as an undifferentiated document. For background on the format and its support for digital signatures, see RFC 8118.
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Adobe’s Node.js electronic seal quickstart describes applying an electronic seal to an invoice PDF through PDF Services and advises storing credential files safely. Applying a seal is a separate workflow; it does not recover the historical PDF bytes or demonstrate that a new rendering matches them.
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Use a suitable hash algorithm
Node.js documentation warns that MD5 and SHA-1 are no longer acceptable where collision resistance is required, including digital signatures. For file comparisons, choose an appropriate current algorithm such as SHA-256, and consult guidance for the Node.js and OpenSSL versions deployed in your environment. See Node.js guidance on weak or compromised algorithms.
Keep legal and technical conclusions distinct
A successful digest comparison answers a narrow technical question: do these examined bytes match the bytes represented by the recorded digest? It does not decide whether the invoice is accurate, who created it, or whether it is legally sufficient evidence. Retention periods and admissibility requirements depend on jurisdiction; without a specified jurisdiction, no universal retention period or legal conclusion can be given.
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