You sign a PDF and the file gets... slightly different. A signed PDF carries more than just the visible signature mark. Here's what's actually in there.
The visible mark
The drawn or typed signature on the page is just an image (for electronic signatures) sitting in an annotation layer or, after flattening, baked into the page content. Visually, it's identical to a scribble. Functionally, it's a marker showing 'I agreed'. Sign PDF creates this in seconds. For most everyday contracts, the visible mark plus an intact file is enough legally.
Signature metadata
Many signing tools record metadata alongside the visible mark: time of signing, IP address, signer name, sometimes a unique transaction ID. This lives in the PDF's metadata, not on the visible page. If a dispute arises, this is the data lawyers reach for. Not every electronic signature includes this; tools vary. For higher-stakes contracts, use a tool that explicitly records audit data.
Digital signatures: cryptographic seals
Digital signatures (different from drawn electronic signatures) add a cryptographic certificate. The certificate proves who signed and that nothing changed since. Your viewer checks the seal automatically — a green tick means valid, a red warning means tampered or expired. Used for legal, government, and finance documents. Stronger than electronic, more friction to set up. For everyday use, electronic + flatten + email is fine.
FAQ
Can I edit a signed PDF?
You can, but doing so usually invalidates digital signatures. For electronic signatures, editing is possible but unethical — the signed version is the agreed version. Always treat signed PDFs as final.
How do I verify a digital signature?
Open in a viewer that supports verification (Adobe Reader, Foxit). The viewer checks the certificate chain and tells you valid or invalid. If you can't verify, ask the signer for a verification certificate.
Does flattening change the signature?
It bakes the visible mark into the page so it can't be removed as an annotation. Doesn't affect cryptographic digital signatures, which seal the whole file. For drawn signatures, flatten before sending.
Signed PDFs hold more than meets the eye. Sign yours properly, flatten, and store.