How to Sign a PDF Document

Signing a PDF is not one thing. There are at least three different legal and technical mechanisms, and the right choice depends entirely on what the document is for.

The three kinds of signatures

1. Electronic signature (image on page)

A drawn, typed or uploaded signature image placed on the PDF. Legally valid for many contracts under the ESIGN Act (US), eIDAS (EU) and similar laws elsewhere. OmniPDF's Sign PDF tool produces this kind of signature.

Visually it looks like a signature. Technically it is an image baked into the page. Anyone with the file can verify that it "looks like a signature was placed there" — but there is no cryptographic proof that you actually signed it, no way to detect tampering, and no trusted third party confirming your identity.

2. Digital signature (cryptographic)

A digital signature uses public-key cryptography. Your private key signs the document's hash. The recipient can verify that the document has not been modified since signing, using your public key. Digital signatures include a certificate chain — a way to prove that the signer is who they claim to be, verified by a Certificate Authority.

Digital signatures are much stronger than image signatures. They provide integrity (any change invalidates the signature), authenticity (proves the signer's identity) and non-repudiation (the signer cannot deny signing).

3. Qualified digital signature

In the EU (under eIDAS) and some other jurisdictions, a "qualified" signature has added legal weight because it uses a government-approved hardware token or remote signing service. This is the strongest form and required for certain high-value contracts.

Which to use

• Everyday contracts, invoices, forms: electronic signature image is fine.
• Employment offers, NDAs, medium-value contracts: digital signature from a service like DocuSign, Adobe Sign or Dropbox Sign.
• Government filings, property deeds, high-value contracts: qualified digital signature.

Signing with OmniPDF

The Sign PDF tool produces an electronic signature image. Here's how it works:

1. Open Sign PDF.
2. Add your PDF.
3. Draw your signature on the canvas with a mouse, trackpad or finger. Or upload a PNG/JPG of your signature.
4. Set the position (X, Y in points from the top-left) and size.
5. Click Apply signature. The signature is flattened into the page and downloads as a PDF.

What OmniPDF's signature does and does not do

Does:

• Places a visible signature on the page.
• Preserves the rest of the PDF content unchanged.
• Is legally valid in most everyday contexts under ESIGN, eIDAS and similar laws.

Does not:

• Provide cryptographic proof of who signed.
• Detect tampering after signing.
• Include a timestamp from a trusted authority.
• Replace a qualified signature for regulated agreements.

When to use a dedicated signing service instead

Use DocuSign, Adobe Sign, Dropbox Sign or your national eID provider when:

• The contract exceeds a certain value (varies by jurisdiction and industry).
• The recipient requires cryptographic proof of authorship.
• You need an audit trail (when signed, from which IP, with which device).
• The document is subject to regulatory audit.
• You are signing a will, deed or other document that requires notarisation.

Signature best practices

• Use a clear, legible signature. Obscure signatures are legally weaker if they cannot be linked to you.
• Sign the right page. Signature pages are usually the last page or a designated block. Check before signing.
• Add date and location. Many contracts require these for validity.
• Keep a copy. Save both the unsigned original and the signed copy.
• Watch for tampering. If you receive a signed document, check that the content has not been modified after signing. A simple check: compare the text of the received file against the text of the file you signed. If they differ, something changed.

Verifying a signature

OmniPDF's Verify Digital Signature tool inspects the signature dictionary of a PDF and shows signer, reason, location and subfilter info. This is structural inspection only — it does not perform full PKI validation. For that, use Acrobat Pro or your national eID provider.

Privacy

Signing happens in your browser. Your signature image never leaves your device.