Sign a PDF Document

Under the hood, the process of signing a document via PSPDFKit Document Engine is divided into three phases:

  1. PSPDFKit Document Engine prepares the document for a signature, adding an invisible form field that will contain the signature value.

  2. PSPDFKit Document Engine then contacts an external signing service you’re responsible for setting up, which will provide a compliant signature value.

  3. PSPDFKit Document Engine applies the returned signature to the document and saves it, storing the final file as an asset associated with the document and the used Instant layer.

This architecture ensures that PSPDFKit doesn’t need access to the private key that ultimately will be used to produce the signature value, leaving you complete freedom to choose which strategy to use to manage its lifecycle and security.

The Signing Service

The signing service is a network service that you’re responsible for maintaining and operating.

It needs to expose a single HTTP endpoint of your choice that receives all the information required to calculate a compliant digital signature, and it should return a DER PKCS#7 container that can be set as a value of the digital signature field.

For example, say you want to sign a document with the ID my-document-id via the Server API:

POST http://localhost:5000/api/documents/my-document-id/sign
Authorization: Token token="<secret token>"
Content-Type: application/json

  "signingToken" : "custom-token"

The request accepts an optional signingToken string parameter, which will be forwarded to the signing service in the exact same shape.

You can use it to pass a token that can be used to verify the authenticity of the signing request or to provide identity information about the user applying the signature.

The signing endpoint will receive a request with the following schema:

POST http://signing-server:6000/sign
Content-Type: application/json

  "encoded_contents" : "CkVudW1lcmF0aW5nIG9iamVjdHM6IDExLCBkb25lLgpDb3VudGluZyBvYmplY3RzOiAxMDAlICg...",
  "digest" : "aab7fe5d814e7e8048275d19693435013727ee8002b85ba8edc29321fc2edfc9",
  "signing_token" : "custom-token"

In the example above, we assume that the signing service can be accessed at http://signing-server:6000/sign.

The endpoint will receive a JSON-encoded POST request containing:

  • The Base64-encoded contents of the file to sign. This represents the portion of the PDF document covered by the digital signature, minus the byte range that will contain the signature itself. Note that since it’s Base64-encoded, you’ll need to decode it before signing it.

  • The digest for the contents to be signed (with the hash calculated before the contents are encoded to Base64). If your language and encryption libraries support it, you can perform the signature operation using the hash as the signature contents. In such a case, make sure you configure PSPDFKit Document Engine to use at least sha256 as its hashing algorithm.

  • The signing token, forwarded from the previous step.

For more details, refer to our signing service reference implementation on GitHub.

We recommend setting up the signing service as a container on the same network as PSPDFKit Document Engine and without external network access to guarantee fast, consistent performance and better security.

Once the signing service is up and running, you can configure PSPDFKit Document Engine to use it by setting the SIGNING_SERVICE_URL to the signing service endpoint, e.g. http://signing-service:6000/sign. For more information on configuration and customization, refer to our configuration guide.

Applying a Signature

To digitally sign a document, use the /api/documents/:document_id/sign endpoint. Refer to the API Reference for the full list of available options.