PDF authenticity
A label hidden with steganography identifies the brand or recipient of each file. It can only be extracted with Verify.
Sign PDFs with steganography and verify their authenticity
Hide your own label inside the file, remove its metadata and protect it against changes. Everything happens in your browser, free and with no sign-up.
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Upload your PDF
Drag your file here or click to select it.
Drag the PDF you want to sign here
Or click to load it from your device
Supported format: PDF · Up to 80 MB
Or click to load it from your device
Supported format: PDF · Up to 80 MB
Free and no sign-up
Reading your PDF…
One moment, we are checking the file on your device.
Review your file
Residual information found
All of this residual information will be removed from the file. The text, images and look of the document do not change.
Processing your PDF
Keep this tab open until it finishes.
Your PDF is signed and protected
DownloadThe random protection key was discarded at the end and is not stored anywhere.
Hidden signature
Fingerprints of the final file
Keep these fingerprints: if a single byte of the file changes, they change completely.
Verification result
File fingerprints
Current protection
We couldn't process the file
100% private: your PDF is processed on your device and never uploaded to any server.
Four guarantees for every PDF
The hidden signature, the fingerprint and the encryption work together: they identify the file, reveal changes and limit editing.
Integrity with hash fingerprints
SHA-1, SHA-256 and SHA-512 fingerprints are calculated, and the signature stores a fingerprint of the content to detect PDF tampering.
Confidentiality
Metadata removal deletes the author, software, dates, application data and old versions the file carries without you knowing.
PDF protection
AES-256 encryption with a random key that is never stored: the reader blocks editing and content extraction.
How it works in 3 steps
- 1
Upload your PDF
Choose Sign PDF, select the file and review the summary of residual information that will be removed. Then type the short string that will be your hidden signature.
- 2
Your browser does the work
It cleans the metadata, hides the signature with steganography, protects the file with a random key and calculates the SHA-1, SHA-256 and SHA-512 fingerprints. Nothing is uploaded to the internet.
- 3
Download and verify
You get the PDF with the same name plus today's date. Whenever you want to check it, load it in Verify PDF: the signature is extracted and the tool confirms the content has not changed.
Use cases
For anyone who delivers documents and may need to prove where they came from later.
Reports and client deliverables
Mark each delivery with the client and project name. If a report circulates without permission, you will know where it came from.
Confidential documents
Use a different signature for each recipient. If the file leaks, the extracted signature shows which copy it was.
Contracts and proposals
Check that the PDF you get back is the one you sent, with no changes to clauses, amounts or dates.
Expert reports, theses and academic work
Deliver reports and papers with their hash fingerprint and a hidden signature that proves the original version.
Invoices and quotes
Detect changes to amounts or items after sending, without relying on the client keeping your original.
Portfolios, CVs and catalogs
Remove author and software metadata before publishing, and leave your own mark that proves where the file came from.
Frequently asked questions
What is steganography in a PDF?
It is the technique of hiding information inside a file without it being noticed (PDF steganography). FirmaDigitalPDF stores your signature in the internal spacing of the page content: the document looks and prints the same, and the signature can only be extracted with the Verify PDF option.
Are my files uploaded to a server?
No. All processing happens in your browser. The PDF never leaves your device and no copy is kept.
Is this a certificate-based digital signature?
No. It does not use certificates or signature images. It is a hidden mark that identifies where the file came from and reveals later changes. It does not replace a qualified electronic signature when a regulation requires one.
Which metadata is removed?
Title, author, subject, keywords, creator and producer software, creation and modification dates, XMP metadata, private application data, comment authors, the document ID and objects left over from earlier versions. Data inside embedded images, such as a photo's EXIF, is not changed.
How is the file protected against editing and copying?
The file is encrypted with AES-256 and a random owner key that is discarded at the end, so nobody keeps it. PDF readers respect the permissions and block editing and copying of content. Some specialized tools can bypass permission restrictions, which is why the hidden signature and the content fingerprint add a second check.
How does it detect that a PDF was tampered with?
The hidden signature includes a fingerprint of the pages, images and fonts (PDF tamper detection). If anything changes after signing, Verify PDF reports it. The file's SHA-1, SHA-256 and SHA-512 fingerprints also let you compare it byte by byte with the original. If another program rewrites the page content, the signature may be lost and the result will be “no signature found”.
What is a hash fingerprint and what is it for?
It is a unique code calculated from every byte of the file. If a single byte changes, the code changes completely. Keep the SHA-256 or SHA-512 fingerprint of the final file and compare it whenever you need to check that it was not altered.
Can I sign a PDF that is already signed?
Yes. If you load a PDF that came out of this tool, it opens without a password, the previous signature is replaced with the new one and the file is protected again.
Which PDFs cannot be processed?
Those that ask for a password to open: remove the protection with your password and try again. PDFs that only have permissions (AES-256, AES-128 or RC4) open without problems. The size limit is 80 MB, although the result depends on your device's memory.
Who owns the code and can I copy it?
The source code of this site is owned by Audit4Web.com. It is obfuscated, and reverse engineering, copying and reuse without written permission are prohibited.