How to digitally sign a PDF (and what 'digitally' actually means)

People use 'digitally sign' and 'electronically sign' interchangeably, but they refer to different things. An electronic signature is broadly any symbol or process applied with the intent to sign a document — a typed name, a drawn signature, a clicked button. A digital signature is a specific subset that uses public key infrastructure (PKI) cryptography to prove authenticity. If someone asks you to 'digitally sign' a PDF, clarifying which they mean is worth doing.

What a digital signature actually is

A digital signature is a cryptographic certificate issued by a trusted certificate authority (CA). When you apply it to a PDF, the document gets signed with your private key — a mathematical proof that you specifically signed it, and that the document hasn't been altered since. Adobe Acrobat displays this as a blue checkmark with 'Signature is valid' when the certificate is trusted.

Digital signatures in this strict sense are used where the highest level of authentication is required: regulated financial documents, government filings, pharmaceutical approvals, and legal contexts in some EU member states under eIDAS QES (Qualified Electronic Signature) requirements. For most commercial documents — contracts, NDAs, freelance agreements, lease documents — a standard electronic signature with a proper audit trail provides equivalent legal validity at a fraction of the cost and complexity.

How to apply a PKI digital signature to a PDF

To apply a proper PKI digital signature, you need a digital certificate from a certificate authority. These come from providers like DigiCert, GlobalSign, or Comodo, and typically cost £50–£250 per year for a personal certificate. Once you have the certificate, you install it in your operating system's certificate store and apply it via Adobe Acrobat (Tools > Certificates > Digitally Sign) or similar PDF editors that support PKI.

• Obtain a digital certificate from a trusted CA (DigiCert, GlobalSign, Comodo)
• Install the certificate in your system's certificate store
• Open the PDF in Adobe Acrobat Pro
• Go to Tools > Certificates > Digitally Sign
• Draw the signature area, select your certificate, and confirm
• The PDF is now signed with a cryptographic stamp that recipients can verify

When you don't need a PKI digital signature

For the vast majority of commercial signing scenarios — clients signing service agreements, tenants signing leases, freelancers signing contracts — a standard electronic signature with an audit trail is legally equivalent and significantly easier to use. The ESIGN Act and UETA in the US, and eIDAS SES (Simple Electronic Signature) in the EU, all recognise simple electronic signatures as legally valid for most document types.

The audit trail provided by platforms like InkRobin — email, timestamp, IP address, SHA-256 document hash — addresses the same concerns a PKI signature addresses: who signed, when, and whether the document has changed. For most commercial use cases, this is sufficient and considerably more practical.

How to electronically sign a PDF without PKI

Upload the PDF to an e-signature platform. Place a signature field. Sign using a typed name, drawn signature, or uploaded signature image. The platform records the signing event with a timestamp and document hash. Download the signed PDF with the audit trail embedded as a Certificate of Completion. This covers about 95% of signing use cases for businesses and individuals.

Frequently asked questions

• Is a digital signature the same as an electronic signature? No. A digital signature uses PKI cryptography and a certificate. An electronic signature is any digital symbol applied with intent to sign.
• Do I need a digital certificate to sign contracts? For most commercial contracts, no. Standard electronic signatures with audit trails are legally valid.
• What's the difference between Adobe's 'Sign' and 'Digitally Sign'? Adobe's basic signature tool creates an electronic signature (image-based). 'Digitally Sign' under Certificates applies a PKI certificate — these are distinct features.