How to digitally sign PDF documents: Complete guide (2025)

A digital signature works like an ink signature on paper, but it provides stronger security. PDF readers like Nutrient use this embedded data to verify two critical factors:

• Document integrity — no unauthorized modifications
• Author authenticity — confirmation of the signer’s identity

How the integrity of a PDF document is checked

If someone intercepts a PDF contract and changes its terms, digital signatures detect this tampering through hashing. A hash function converts document data into a fixed-size string. Any modification to the document produces a different hash value.

Signing a PDF applies a hash function to the document’s contents and stores the result. But hashing alone isn’t enough — an attacker could modify the document and recalculate the hash. Public-key cryptography prevents this by creating a tamper-proof digital signature of the hash.

Public-key cryptosystems

Traditional encryption requires both parties to share the same secret key beforehand — a security challenge in itself.

Public-key cryptography solves this. For digital signatures, the sender uses a private key to sign the message (or, more efficiently, a hash of it) and shares a public key with the receiver.

Two things must hold: (1) Only the sender controls the private key used to create the signature, and (2) anyone with the corresponding public key can verify that signature against the document’s hash. When these are true, the recipient can be confident the document came from the sender and hasn’t been altered.

To be efficient without compromising security, Nutrient doesn’t digitally sign the entire PDF document; it signs a hash of it. When the recipient gets a signed document, they compute a fresh hash of the received content and use the sender’s public key to verify that the digital signature corresponds to that hash (i.e. that it was created with the matching private key).

If the verification succeeds (the hashes match), the document hasn’t been modified and was created by you. This whole process is automatically supported by Nutrient, so you don’t need to perform these steps manually.

How to validate a digitally signed document with Nutrient

To validate a signed PDF with Nutrient using a set of trusted certificates, create an instance of a PDFSignatureValidator and initialize it with the SignatureFormElement you want to validate. Then call the verifySignature(withTrustedCertificates:) method, like this:

let signatureFormElement = unsignedDocument.annotationsForPage(at: 0, type: .widget)?.first { annotation -> Bool in
    return annotation is SignatureFormElement
}
let validator = PDFSignatureValidator(signatureFormElement: signatureFormElement)
let certificates = try X509.certificates(fromPKCS7Data: certificateData)
let signatureStatus = validator.verifySignature(withTrustedCertificates: certificates)

The signatureStatus variable returns signature details: validity status, creation date, and other metadata.

How to sign a document with Nutrient iOS SDK

To sign a PDF, create a PKCS#12 keystore (PKCS12) that contains your private key and certificate. After that, initialize an instance of a PKCS12Signer with that keystore. Optionally, you can set a reason for the signature:

let keystore = PKCS12(data: p12data)
let signer = PKCS12Signer(displayName: "Your name", pkcs12: keystore)
signer.reason = "Contract agreement"

Sign the document using the modern async API:

// Load the signing configuration.
let configuration = SigningConfiguration(dataSigner: privateKey, certificates: certificates)

// Sign the document.
try await document.sign(formElement: signatureFormElement,
                       configuration: configuration,
                       outputDataProvider: FileDataProvider(fileURL: outputURL))

Display the signed document using a PDFViewController.

Additional resources from Nutrient documentation:

• Validating a digital signature on iOS
• Adding a digital signature to a PDF on iOS

Cross-platform implementation

Digital signatures work across all major platforms. Each Nutrient SDK provides platform-specific optimizations and native integrations.

Web/JavaScript implementation

Nutrient Web SDK provides full support for digital signatures directly in the browser, powered by the Web Crypto API. This enables secure, standards-compliant signing without requiring external plugins or native applications.

You can implement client-side signing for decentralized workflows or connect to server-side signing services when centralized certificate management is required. The SDK is designed for seamless integration into existing web applications and workflows.

Key capabilities

• Browser-native cryptographic operations via Web Crypto API
• PKCS#12 certificate support for enterprise-grade signing
• Real-time signature validation to ensure authenticity and integrity
• Cross-browser compatibility with modern standards
• Flexible integration with both client-side and server-side workflows

Learn more

For implementation instructions, browser compatibility details, and security best practices, refer to our Web SDK digital signatures guide.

Android implementation

Nutrient Android SDK provides secure digital signature capabilities with deep integration into the Android platform. Signing workflows can leverage the Android Keystore system for certificate storage and private key access, as well as external providers for advanced security needs.

Key features

• Integration with Android Keystore via MemorySigner and KeyStore.PrivateKeyEntry
• Support for external signature providers, including hardware security modules (HSMs)
• Biometric signature data capture (e.g. pressure, timing, input method) for enhanced authenticity
• Full support for both Kotlin and Java SDKs

Implementation resources

• Android digital signatures guide
• Android getting started guide
• Digital signing on Android

.NET cross-platform support

Nutrient .NET SDK provides robust digital signature capabilities across desktop, mobile, and server environments. It runs cross-platform on Windows, macOS, and Linux, with support for modern .NET versions (6, 7, and 8) as well as .NET Framework where required.

Platform support

• .NET MAUI (Windows, macOS via Mac Catalyst, iOS, Android)
• WPF and WinForms desktop applications
• ASP.NET Core web applications
• Console and service applications

This breadth of support makes it possible to build secure signing workflows for everything from enterprise desktop apps to mobile solutions and server-side services.

Getting started

Refer to the .NET SDK documentation for platform-specific setup instructions, system compatibility details, and implementation patterns.

Server-side implementation

Server-side signing with Nutrient Document Engine centralizes certificate management and provides enterprise-grade security and scalability for digital signature workflows.

Enterprise benefits

• Centralized certificate storage and management for compliance and security
• Hardware security module (HSM) integration, including AWS CloudHSM and third-party providers
• Audit trail and compliance logging to meet regulatory requirements
• High-volume batch processing for large-scale document workflows
• API-based integration via Document Engine and the Digital Signatures API for seamless system connectivity

Learn more

• Server-side digital signatures guide
• Digital signature integrations overview
• Nutrient Processor API documentation

Legal compliance and regulatory frameworks

Digital signatures must meet specific legal and regulatory requirements, depending on your industry and jurisdiction, to be enforceable. Nutrient SDKs are designed to help you build compliant workflows across regions and sectors.

eIDAS Regulation (European Union)

The Electronic Identification, Authentication and Trust Services (eIDAS) regulation establishes the framework for electronic signatures in the EU:

US electronic signature laws

ESIGN Act (federal)

The Electronic Signatures in Global and National Commerce (ESIGN) Act establishes the federal framework for electronic signatures:

• Intent to sign – Signer must demonstrate clear intent
• Consent to electronic records – Parties must agree to an electronic process
• Record retention – Signed records must remain accessible and reproducible
• Attribution – Signature must be attributable to a specific individual

UETA (state level)

The Uniform Electronic Transactions Act (UETA) complements ESIGN at the state level:

• Adopted by 48 US states
• Technologically neutral
• Requires authentication of signatures
• Emphasizes preservation of record integrity

Industry-specific requirements

FDA 21 CFR Part 11 (pharmaceutical)

The FDA’s 21 CFR Part 11 governs the use of electronic records and signatures in regulated industries:

Financial services

Industries like banking and payments often require compliance with frameworks such as KYC, AML, Sarbanes-Oxley, and PCI DSS. These are common considerations for secure digital signature workflows.

Signature type comparison

Choosing the right signature type depends on security and compliance requirements:

When to use each type

Security best practices

Implementing digital signatures securely requires careful handling of certificates, keys, and validation processes. Nutrient SDKs and Document Engine provide built-in tools to support trusted, legally binding signature workflows, but correct setup and governance are essential.

Certificate management

Certificate lifecycle

Proper certificate management involves several phases, outlined below.

Generation and enrollment

• Create certificate signing requests (CSRs) with strong key pairs (RSA 2048+ or higher)
• Obtain certificates from a trusted Certificate Authority (CA), ideally AATL-backed for production use
• Install issued certificates with secure storage practices

Validation and monitoring

• Validate certificate chains against trusted root certificates
• Handle untrusted, expired, or self-signed certificates appropriately
• Provide a trusted root store (supported in Document Engine) for verification
• Use revocation checking (CRL or OCSP) as a best practice to ensure certificates haven’t been compromised

Renewal and rotation

• Renew certificates proactively before expiration
• Rotate keys regularly to mitigate long-term risk
• Deploy updates through controlled, auditable processes

Resources

• Preparing certificates for signing
• Providing trusted root certificates

Key storage security

• Hardware security modules (HSMs) — Supported through integrations such as AWS CloudHSM for high-assurance key storage
• Centralized key management — Available via the Document Engine for server-side signing and enterprise workflows
• Access controls — Use role-based permissions and audit logs for certificate and key usage
• Strong keys — Always use secure algorithms (RSA 2048+, ECDSA, or equivalent) for cryptographic operations

Resources

• Key PDF security features
• Best practices for managing digital signatures

Conclusion

Digital signatures secure your PDFs against tampering and verify signer identity, making them essential for compliance and trust. With Nutrient’s cross-platform SDKs and server-side tools, you can implement legally binding signatures on Web, mobile, .NET, and enterprise systems.

Get started with Nutrient’s digital signature SDK and free trial to build secure, compliant workflows today.

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