PDF Signature Field
A special AcroForm field type (/Sig) that marks where a digital or electronic signature should be applied in a PDF document — defining the signature's visual position, size, and appearance, as well as the cryptographic parameters for the signing operation.
A PDF signature field is a specialised AcroForm field (field type /Sig) that designates a location on a PDF page where a digital or electronic signature should be applied. Unlike other form fields that store text or checkbox values, a signature field stores cryptographic signature data — a hash of the document content signed with the signer's private key — or, in simpler workflows, references a visual signature appearance. Signature fields can be placed anywhere on a PDF page and can be locked to prevent modification after signing.
Technical structure of signature fields
A PDF signature field is an AcroForm field widget annotation with /FT (field type) set to /Sig. Its key dictionary entries:
<strong>/V</strong> — The signature value dictionary, populated when the field is signed. Contains: - <code>/Type /Sig</code> — identifies this as a signature - <code>/Filter /Adobe.PPKLite</code> (or equivalent) — the cryptographic filter used - <code>/SubFilter /adbe.pkcs7.detached</code> (or <code>/ETSI.CAdES.detached</code> for PAdES) — the signature format - <code>/ByteRange</code> — the byte ranges of the document that were signed (all bytes except the signature itself) - <code>/Contents</code> — the DER-encoded PKCS#7 signature blob - <code>/Cert</code> or <code>/Certs</code> — the signer's certificate chain - <code>/M</code> — the signing time (may be the signer's local time; a timestamp token is more reliable)
<strong>/Lock</strong> — Specifies which fields are locked after the signature is applied. A certifying signature can lock all remaining fields; an approval signature can lock specified fields only. This is how multi-party signing workflows control which parts of the form each signer can fill.
<strong>/SigFieldLock</strong> — Referenced by /Lock; defines the locked field set.
<strong>/AP</strong> — The appearance dictionary, which defines how the signature looks visually on the page. A signature field can have a visible appearance (showing the signer's name, date, a signature image, and/or a seal) or be invisible (storing only the cryptographic data without visible rendering on the page).
The visual appearance and the cryptographic data are completely independent — a PDF can have a beautifully rendered signature appearance with no cryptographic validity, or a cryptographically strong signature with an invisible (no visual appearance) field. Both are valid uses of the signature field type for different purposes.
Certification vs approval signatures
PDF distinguishes two types of signatures with different semantics:
<strong>Certification signature (/DocMDP)</strong> — Applied first by the document author (typically the form creator). A certification signature can specify what modifications are permitted after it is applied: - Level 1: No changes permitted (fully locked) - Level 2: Form fields and annotations may be added; content locked - Level 3: Form fields, annotations, and digital signatures may be added; page content locked
This allows a form issuer to certify the document's integrity while permitting recipients to fill fields and sign. The certifying signature remains valid through these permitted modifications.
<strong>Approval signature (/FieldMDP)</strong> — Applied by subsequent signers (recipients, approvers). Each approval signature covers the document state at the time of signing. Multiple approval signers can each add their signature; the PDF grows through incremental updates, with each signer's signature validating their specific portion of the signing history.
The combination of certification + approval signatures is how multi-party signing workflows are implemented: the form creator certifies (locks page content), then signers 1, 2, and 3 each apply approval signatures as they fill their sections. <a href="https://www.docusign.com/how-it-works/electronic-signature/pdf-signing" target="_blank" rel="noopener">DocuSign's PDF signing documentation</a> describes how their platform implements this model.
Visible vs invisible signature fields
Signature fields can be visible (rendered on the page with a visual appearance) or invisible (containing only the cryptographic data, no visual footprint on the page).
<strong>Visible signature fields</strong> are typically used for: - Consumer and business contracts where a visual signature is expected (resembling the appearance of a pen signature on paper) - Document approval workflows where seeing the signature on the page confirms the document is signed - Regulatory contexts requiring a visible signature block
A visible signature field typically shows: the signer's name (from the certificate), the signing date/time, the company name, and optionally a scanned signature image or organisation seal. The layout is defined by the /AP dictionary using PDF drawing operators.
<strong>Invisible signature fields</strong> are used for: - Document integrity certification where the signature is meant to prove authenticity without cluttering the page - Back-end automated signing in document generation pipelines - Certifying signatures on forms that will be visually signed by recipients later
Invisible signature fields are fully cryptographically valid — they provide the same integrity guarantees as visible fields. The absence of a visible signature badge does not mean the document is unsigned; always check the Signatures panel in <a href="https://get.adobe.com/reader/" target="_blank" rel="noopener">Adobe Reader</a> (View > Show/Hide > Navigation Panes > Signatures) for cryptographic signature status.
Creating and validating signature fields
<strong>Creating in Adobe Acrobat Pro</strong> — Tools > Prepare Form > add a Signature field widget. Set the /Lock properties, appearance, and whether it is visible or invisible.
<strong>Programmatically</strong> — <a href="https://itext.com/" target="_blank" rel="noopener">iText</a> (Java/C#) has comprehensive PdfSignatureAppearance and PdfSigner APIs that support certification signatures, approval signatures, LTV, and PAdES conformance levels. <a href="https://pypdf.readthedocs.io/" target="_blank" rel="noopener">pypdf</a> supports basic signature field creation; for production cryptographic signing, <a href="https://pyhanko.readthedocs.io/" target="_blank" rel="noopener">pyHanko</a> (Python) is the most capable open-source option for PAdES-compliant PDF signing.
<strong>Validating</strong> — <a href="https://get.adobe.com/reader/" target="_blank" rel="noopener">Adobe Reader</a> is the standard tool for validating PDF signatures: the Signatures panel shows each signature, whether it is cryptographically valid, whether the document has been modified since signing, and the certificate trust chain. The <a href="https://dss.esig.europa.eu/validation/detailed-report" target="_blank" rel="noopener">EU DSS validation service</a> validates PAdES signatures against the EU Trusted List for eIDAS compliance.
Frequently asked questions
What is a PDF signature field?
A PDF signature field is a special form field (/FT /Sig in the AcroForm dictionary) that marks where a digital signature should be applied. It can store cryptographic signature data (certificate-based digital signature) or reference a visual signature appearance. Signature fields can be visible (showing the signer's name and date on the page) or invisible (containing only cryptographic data without visual rendering).
What is the difference between a certification signature and an approval signature?
A certification signature (/DocMDP) is applied first by the document creator and specifies what modifications are permitted afterward (none, form fills only, or form fills and additional signatures). An approval signature is applied by subsequent signers and covers the document state at signing time. Together they enable multi-party signing workflows where a form creator certifies the document and multiple signers each apply their approval signatures.
How do I check if a PDF has a valid digital signature?
In Adobe Reader: View > Show/Hide > Navigation Panes > Signatures. The Signatures panel shows all signature fields and their validity status — green checkmark (valid, unmodified), yellow warning (valid but certificate issues), or red X (invalid or document modified). For PAdES signatures under eIDAS, use the EU DSS validation service for authoritative compliance verification.
Does adding a visual signature appearance make a PDF signature legally stronger?
No. The legal validity of a PDF digital signature comes from the cryptographic binding — the certificate, the hash of the document content, and the trust chain. A visible signature appearance (showing the signer's name and date on the page) is a convenience for human readers; it does not add cryptographic strength. An invisible signature field with a valid certificate is cryptographically equivalent to a visible one.