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This applies a real cryptographic signature: a detached CMS SignedData structure computed over the document's bytes and embedded in the file, which Acrobat and other compliant readers check when the document is opened. Unlike a drawn signature it establishes two things - that the holder of a particular certificate signed, and that not one byte has changed since.

When the recipient will verify rather than merely look: a tender submission, an invoice under an e-invoicing rule, a document going into an archive that requires signed originals, or anything where a later argument about whether the file was altered is a real possibility.

วิธีทำงาน

1. Drop two files onto this page: the PDF, and your certificate as a .p12 or .pfx file. The order does not matter - the PDF is recognised by its contents.
2. Type the passphrase that protects the certificate. It is used in your browser to unwrap the private key and is never sent.
3. Fill in the reason and the location if your recipient expects them. Both are recorded inside the signature and shown by readers that display signature details.
4. Leave visible on to draw a signature block on the page you choose, or turn it off for a signature that exists only in the file's structure.
5. Press Sign with certificate, then open the result in Acrobat to see what a recipient's reader will report.

Every online signing service asks you to upload the one file you should never upload. Here the .p12 is read in the page and parsed inside a worker that holds nothing else, and the worker is terminated when the job finishes - the thread is discarded rather than a variable cleared, so nothing of the key survives into a later job.

A PDF signature covers the byte ranges its /ByteRange array names, and that array has to be written into the file in place, after space for the signature has been reserved. Re-serialising the document afterwards moves every offset and invalidates what was just computed - the mistake naive implementations make, and the reason some signed PDFs report as broken the moment they are opened.

A self-signed certificate produces a valid signature that readers report as validity unknown, because they have no reason to believe the name on it. That is not a failure: it says the mathematics checks out and the identity does not. A certificate issued by an authority your recipient's reader already trusts is what turns that into a green tick.

There is no trusted timestamp and no revocation checking, because both need a network service that this tool never contacts. The signing time recorded is your own computer's clock, which a strict recipient should treat as a claim rather than proof. An encrypted PDF is refused for a related reason: a signature computed over ciphertext is not something a reader will validate.