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Digital Signatures Explained

by Hung Nguyen

You can also read this article in German, Spanish, French, Indonesian, Italian and Portuguese.

Smallpdf can help you create a digital signature and digitally sign your documents within minutes. You can also request verifiable signatures.

Smallpdf is the most popular online PDF software. We offer a standard electronic signing tool for free users, and digital signing for anyone with a Pro subscription. Below we’ll explain both options, along with how to request signatures.

How To Create a Digital Signature Online

  1. Upload the document you want to sign to our eSign tool.

  2. Click “Your Signature” and create your signature.

  3. Drag and drop the signature onto the document.

  4. Click “Finish and Sign,” and then “Save” to download your signed PDF.

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Create a digital signature and sign your document in seconds!

You can use our eSign tool to sign any PDF document. If you have electronic documents in other formats, you can convert them to PDF before sealing them with a digital signature.

To streamline the digital signing process, you can create online signatures on PCs, mobile devices, or even tablets using the above method.

How to Request a Verifiable Digital Signature

  1. Open your file with our eSign tool.
  2. Click “Invite signers” and fill out their names and email addresses.
  3. If needed, add text and date fields or checkboxes.
  4. “Review & Send” the document. You can also add a personalized message.
  5. Hit “Finish & Send” and your recipients will receive a signature request.

After the last person has signed the document, Smallpdf will seal the document with a digital signature.

We’ll keep your digital document online for 14 days, and will send reminder emails to notify everyone to download the final version of the document.

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How to request a digital signature from someone else (via email)

What Is a Digital Signature?


A digital signature is a cryptographic process where a large unique number is calculated based on the document’s contents and the attached certificate, which contains the name of the signee. The number that is calculated for the document can be verified with many PDF readers or software.

Whenever a digital signature is added to the document it’s sealed with this unique number that is attached to that particular signature. This means that all the information that was added by the signee, including the signature itself, becomes read-only and uneditable. Additional signees can add their digital signatures and additional information on top of the existing signatures without tampering with and invalidating them. The document is then sealed again following the addition of every new signature.

The Embedded Timestamp of Digital Signatures


Besides our digital signature, we also add a timestamp that’s signed and can’t be forged. The timestamp records the exact time when a user places the signature on the document. For Smallpdf users, this is when Smallpdf processes the file online.


Signing on mobile devices is becoming the norm in digital workplaces

How Do Smallpdf Digital Signatures Work?


When you sign a document using our tool, we provide a Simple Electronic Signature that includes a Long Term Validation- (LTV-) enabled timestamp. Specifically, we use a certificate issued to us, Smallpdf AG, by a Trust Service Provider, Global Sign, to put a digital signature on your document.

You can view the digital signature’s properties with most PDF readers, which should display the validity summary, containing:

  • whether the signature is valid
  • the time of signing
  • whether it’s been modified since the time of signing
  • There should also be a “Signer Info” section, where you can display our certificate. It should display our company name, the validity length of our digital signatures, and the name of the issuer of our digital signature—Global Sign.

There is also a “Details” tab to show further technical information about our certificate. It should include the authority info access, authority key identifier, certificate policies, CRL distribution points, key usage, SHA1 digest of public key, serial number, signature algorithm, subject key identifier, etc.

If you’d like further instructions on how to view this information, we’ve got you covered—just head to How To Verify a Digital Signature.

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How to check the validity of a digital signature

Free Digital Signatures


If you’d like to test out the eSign tool, you can create an electronic signature right away, for free. It’s always best to check in with your local laws on the legality of electronic and digital signatures in your country.

Unsure which type of signature you need? You can check out our article on the difference between electronic and digital signatures to help figure it out.

To get a verified signature, you can sign up for a free 7-day trial to Smallpdf Pro. We’ll also remove all ads, store your online signatures for later use, and give you unlimited access to 20+ other useful PDF tools!

Hung Nguyen
Hung Nguyen
Senior Growth Marketing Manager @Smallpdf