Introduction
PDF stands for Portable Document Format. It is a widely used file format designed to preserve a document’s appearance across devices and operating systems.
A PDF can contain text, images, forms, links, and other document content. Because recipients can usually open it with common software, PDF is a practical format for sharing agreements, invoices, reports, and other business documents.
A PDF file is not automatically a signed or legally binding document. Its legal effect depends on the electronic signature it contains, the signing process, and the applicable law.
Legal status and compliance
PDF is a document format, not a signature level. A PDF can be unsigned, signed with a basic electronic signature, or signed with an advanced or qualified electronic signature.
Under the EU eIDAS Regulation, an electronic signature cannot be denied legal effect solely because it is electronic. A qualified electronic signature has the equivalent legal effect of a handwritten signature across the EU.
For a signed PDF, the key questions are who signed, how the signer was identified, whether the certificate is trusted, and whether the signed content has changed. The file extension alone does not answer these questions.
Technical overview
A PDF is a self-contained document file that can combine content, fonts, layout instructions, images, forms, and metadata. This helps the document retain its intended appearance when it is opened on different devices.
A typical PDF may contain:
Text, images, and page layout information
Embedded fonts and colour profiles
Interactive forms, links, and annotations
Metadata, such as title, author, and creation details
An embedded electronic signature, where the document has been signed
PDF electronic signatures and PAdES
PAdES stands for PDF Advanced Electronic Signatures. It is an ETSI standard family for electronic signatures embedded directly in a PDF file.
With PAdES, the signed PDF and its signature are contained in one file. This makes the document easy to share, store, and validate without a separate signature container.
A PAdES signature can include information used for validation, such as the signer’s certificate, signing time, and long-term validation data. The exact information available depends on the signature profile and the signing service.
After a PDF is signed, changes to the signed content can invalidate the signature. Some workflows may allow limited actions, such as filling in designated form fields or adding another signature; always check the validation status after any change.
How to open and verify a signed PDF
Open the original PDF in a compatible application or service that supports PDF signature validation, such as Adobe Acrobat or Agrello.
Check the signature panel or validation result to confirm:
The signer’s identity and certificate information
Whether the signature is valid and the signed content is unchanged
The signing time or timestamp, where available
Any warnings about certificate trust, revocation, or document changes
Practical tips
When working with a signed PDF:
Keep the original signed PDF file
Do not edit, print-to-PDF, or re-save the document after signing, as it makes the signature invalid.
Share the original file rather than a screenshot or a copied version
Store the signed PDF with its related business records
Further reading