Digital signatures are the electronic equivalent of handwritten signatures, and for PDF documents specifically, they're a powerful tool for authenticating documents, proving authenticity, and ensuring content hasn't been altered. This guide explains everything you need to know.
Electronic Signature vs Digital Signature
These terms are often confused but they're technically different:
- Electronic Signature: Any electronic symbol attached to a document as a signature — could be a typed name, a scanned image of your signature, or a finger-drawn signature. Broadly legally binding under ESIGN Act (US) and eIDAS (EU).
- Digital Signature: A specific type of electronic signature that uses cryptographic technology (PKI — Public Key Infrastructure) to provide stronger authentication and tamper-evidence. Certifiably proves who signed and that the document hasn't changed since signing.
How Digital Signatures Work
Digital signatures use asymmetric cryptography:
- The signer has a digital certificate containing a private key (kept secret) and a public key (shared)
- When signing, a mathematical hash of the document is created
- This hash is encrypted with the private key — this becomes the signature
- Recipients decrypt the signature with the public key to verify the hash
- If the document has changed since signing, the hash will not match — the signature is invalid
This provides two guarantees: authentication (only the holder of the private key could have signed) and integrity (the document hasn't changed since signing).
Types of PDF Signatures
Approval Signatures
Document approval at a specific point in time. Multiple people can apply approval signatures to a document, showing each person's review and approval.
Certifying Signatures
Applied by the document's author to certify its contents. Certifying signatures can specify what changes (if any) are allowed after certification — none, form filling only, or form filling and commenting.
Legal Validity
Digital signatures are legally binding in most countries:
- United States: ESIGN Act (2000) and UETA give electronic signatures the same legal weight as handwritten signatures
- European Union: eIDAS regulation (2016) recognizes multiple levels of electronic signatures
- India: Information Technology Act 2000 recognizes digital signatures
How to Sign PDFs for Free
Several free options exist for adding signatures to PDFs:
- Adobe Acrobat Reader (Free): Fill & Sign tool allows placing signature images or drawing signatures
- Smallpdf: Basic electronic signature capability in free tier
- DocuSign (Free trial): Professional e-signature service with limited free usage
- SignNow: Free tier available for basic document signing
- Apple Preview (macOS): Built-in signature creation and PDF signing
When You Need Qualified Digital Signatures
For legally sensitive documents (high-value contracts, government filings, legal agreements), qualified digital signatures may be required. These are backed by qualified trust service providers (QTSPs) certified under eIDAS in Europe, providing the highest level of legal assurance.
Frequently Asked Questions
Yes. Under the US ESIGN Act, EU eIDAS regulation, and similar laws in most countries, properly executed digital signatures are legally equivalent to handwritten signatures.
Qualified digital signatures are issued by accredited trust service providers and have the highest legal status (equivalent to handwritten signatures in the EU). Standard digital signatures are technically sound but have lower legal standing in some jurisdictions.
Yes. Adobe Acrobat Reader's free Fill & Sign tool lets you add signature images or draw signatures on PDFs. For legally binding digital signatures with certificates, you need additional software.
Certifying signatures can lock a document to prevent further changes (or allow only specified changes like form filling). Approval signatures may not lock the document.