Ký số PDF bằng chứng thư X.509 miễn phí trực tuyến

What a digital signature actually is

A drawn signature is a picture pasted onto a page — it proves nothing and any editor can move or remove it. A digital signature is a CMS (PKCS#7) cryptographic structure embedded in the file itself: it binds your X.509 certificate to a hash of the exact bytes you signed. Change one byte afterwards and every PDF reader flags the document as modified.

This tool produces that real thing. You load a PKCS#12 bundle (.pfx or .p12) containing your private key and certificate, and the tool writes a signature dictionary with a proper /ByteRange and /Contents into the PDF — the same structure Adobe Acrobat creates. The result opens in Acrobat, Foxit, or any standards-based reader with the signature panel populated.

Visible or invisible

A visible signature also draws an appearance on a page you choose — signer name, reason, date, and optionally a scanned signature image — so a printed copy still shows the mark. An invisible signature embeds the same cryptographic proof with no visual stamp, which is what you want for integrity-only sealing of contracts, invoices, and audit exports.

Reason and location fields are recorded inside the signature dictionary itself, so verifiers see 'Approved for release — Jakarta' rather than guessing.

What you can configure

Every part of the signature is under your control.

Private by construction

Your .pfx never leaves the device. The private key is imported into the browser's WebCrypto engine as non-extractable for the signing operation, the CMS blob is built locally, and the finished file is generated in memory. There is no server round-trip at any point — you can disconnect from the network and signing still works.

Pair this tool with Validate Signature to check incoming signed PDFs, or with Timestamp PDF to add a trusted RFC 3161 timestamp that proves when the signed document existed.