---
title: "PDF digital signature API"
canonical_url: "https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-digital-signature-api/"
md_url: "https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-digital-signature-api.md"
last_updated: "2026-07-06T00:00:00.000Z"
description: "Digitally sign PDF files with Nutrient DWS Processor API. Apply invisible or visible cryptographic signatures, configure signature appearance, and flatten documents before signing."
---

# PDF digital signature API

Use the PDF digital signature API to apply a cryptographic digital signature to a PDF file. Digital signatures help prove document authenticity and detect changes made after signing.

The [`/sign` endpoint](https://www.nutrient.io/api/reference/public/#tag/Digital-Signatures/operation/sign-file) handles digital signing. This differs from the Build API used by most document editing operations.

For signup, pricing, and task-level examples, refer to the [digital signatures API task page](https://www.nutrient.io/api/digital-signatures-api/).

The Processor API digital signature endpoint cryptographically signs PDF files as part of a processing workflow. For human eSignature workflows with recipients, signing sessions, reminders, and signing workflow state, use DWS Signer API.

## Sign a PDF

The following example signs `document.pdf` and writes the signed output to `result.pdf`:

### Shell

### Shell (Windows)

### Java

### C#

### JavaScript

### Python

### PHP

### HTTP

## Create an invisible signature

If you omit the `appearance`, `position`, and `formFieldName` options, the API creates an invisible digital signature. The signature remains cryptographic, and PDF readers that support digital signature validation can validate it.

Use this data object to create an invisible signature:

```json

{
  "signatureType": "cades",
  "cadesLevel": "b-lt"
}

```

Use an invisible signature when you need document integrity and authenticity without rendering a signature appearance on the page.

## Create a visible signature

To create a visible signature, provide a `position` object and an `appearance` object. The `position.rect` array uses PDF points and has the format `[left, top, width, height]`.

Use this data object to create a visible signature:

```json

{
  "signatureType": "cades",
  "cadesLevel": "b-lt",
  "position": {
    "pageIndex": 0,
    "rect": [72, 650, 250, 80]
  },
  "appearance": {
    "mode": "signatureAndDescription",
    "showWatermark": true,
    "showSignDate": true,
    "showDateTimezone": false
  }
}

```

### Shell

Run this request to create a visible signature:

```bash

curl -X POST https://api.nutrient.io/sign \
  -H "Authorization: Bearer $NUTRIENT_API_KEY" \
  -F file=@document.pdf \
  -F 'data={
    "signatureType": "cades",
    "cadesLevel": "b-lt",
    "position": {
      "pageIndex": 0,
      "rect": [72, 650, 250, 80]
    },
    "appearance": {
      "mode": "signatureAndDescription",
      "showWatermark": true,
      "showSignDate": true
    }
  };type=application/json' \
  -o result.pdf

```

## Sign an existing signature field

If the PDF already contains a signature form field, use `formFieldName` to sign that field.

Use this data object to sign an existing signature field:

```json

{
  "signatureType": "cades",
  "cadesLevel": "b-lt",
  "formFieldName": "signature-field",
  "appearance": {
    "mode": "signatureAndDescription",
    "showWatermark": true,
    "showSignDate": true
  }
}

```

If a field with the specified name doesn’t exist, the API can create it at the position specified by `position`. If a signature field with that name already exists, don’t also pass `position`.

## Add a custom signature image

You can include an image in the multipart request and reference its content type in the signature appearance. Supported image content types include `image/png` and `image/jpeg`.

Run this request to add a custom signature image:

```bash

curl -X POST https://api.nutrient.io/sign \
  -H "Authorization: Bearer $NUTRIENT_API_KEY" \
  -F file=@document.pdf \
  -F image=@signature-watermark.png \
  -F 'data={
    "signatureType": "cades",
    "cadesLevel": "b-lt",
    "position": {
      "pageIndex": 0,
      "rect": [72, 650, 250, 80]
    },
    "appearance": {
      "mode": "signatureOnly",
      "contentType": "image/png",
      "showWatermark": true,
      "showSignDate": false
    }
  };type=application/json' \
  -o result.pdf

```

The `/sign` endpoint also accepts a `graphicImage` multipart part for the graphic image used as part of the signature appearance.

## Flatten before signing

Set `flatten` to `true` to flatten annotations and form fields before the API applies the signature. This keeps the document appearance stable and removes editable records before signing.

Use this data object to flatten before signing:

```json

{
  "signatureType": "cades",
  "cadesLevel": "b-lt",
  "flatten": true
}

```

Flattening removes annotations and form fields as editable records. Use it only when the signed output should be treated as a final artifact.

## Sign a password-protected PDF

If the source PDF is password-protected, pass the password in the `pspdfkit-pdf-password` header.

Run this request to sign a password-protected PDF:

```bash

curl -X POST https://api.nutrient.io/sign \
  -H "Authorization: Bearer $NUTRIENT_API_KEY" \
  -H "pspdfkit-pdf-password: document-password" \
  -F file=@protected-document.pdf \
  -F 'data={
    "signatureType": "cades",
    "cadesLevel": "b-lt"
  };type=application/json' \
  -o result.pdf

```

If the password contains characters that HTTP header handling might modify, pass it as `base64:<encoded-password>`.

## Use digital signing after other processing steps

Digital signing should usually be the final step in a document workflow. Apply content-changing operations before signing, such as:

- Merging PDFs

- Splitting or extracting pages

- Rotating pages

- Filling forms

- Flattening annotations or form fields

- Redacting content

- Adding watermarks

- Optimizing or linearizing the document

If you modify a PDF after signing it, the signature may become invalid or show that the document changed after signing.

## Reference

A PDF digital signature request uses the `/sign` endpoint with a multipart body.

```typescript

type CreateDigitalSignature = {
  // Flatten annotations and form fields before signing.
  flatten?: boolean,

  // Name of an existing signature field to sign.
  // If the field does not exist, provide position to create a visible field.
  formFieldName?: string,

  // Position for a visible signature appearance.
  position?: {
    pageIndex: number,
    rect: [number, number, number, number],
  },

  // Visible signature appearance settings.
  appearance?: {
    mode?: "signatureOnly" | "signatureAndDescription" | "descriptionOnly",
    contentType?: "application/pdf" | "image/png" | "image/jpeg",
    showWatermark?: boolean,
    showSignDate?: boolean,
    showDateTimezone?: boolean,
  },

  // Signature format options used by Processor API signing examples.
  signatureType?: "cades",
  cadesLevel?: "b-lt",
};

```

Multipart fields:

```typescript

type SignRequest = {
  // Required PDF input.
  file: File,

  // Optional JSON signing parameters.
  data?: CreateDigitalSignature,

  // Optional watermark image for the signature appearance.
  image?: File,

  // Optional graphic image for the signature appearance.
  graphicImage?: File,
};

```

## Related API reference operations

- Refer to the [digitally sign a PDF file endpoint](https://www.nutrient.io/api/reference/public/#tag/Digital-Signatures/operation/sign-file) API reference to apply a cryptographic digital signature to a PDF file.

- Refer to the [build document endpoint](https://www.nutrient.io/api/reference/public/#tag/Document-Editing/operation/build-document) API reference to prepare a PDF before signing by merging, filling, flattening, redacting, watermarking, or optimizing it.

## Related guides

- Refer to the [PDF form filling API](https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-form-filling-api.md) guide.

- Refer to the [PDF flatten API](https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-flatten-api.md) guide.

- Refer to the [PDF merge API](https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-merge-api.md) guide.

- Refer to the [PDF split API](https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-split-api.md) guide.

- Refer to the [PDF rotate API](https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-rotate-api.md) guide.

- Refer to the [PDF watermark API](https://www.nutrient.io/guides/dws-processor/tools-and-api/pdf-watermark-api.md) guide.

- Refer to the [tools and APIs](https://www.nutrient.io/guides/dws-processor/tools-and-api.md) guide.
---

## Related pages

- [Image-to-PDF API](/guides/dws-processor/tools-and-api/image-to-pdf-api.md)
- [Document-to-image API](/guides/dws-processor/tools-and-api/document-to-image-api.md)
- [DOCX templating API](/guides/dws-processor/tools-and-api/docx-templating-api.md)
- [Import XFDF annotations API](/guides/dws-processor/tools-and-api/import-xfdf-annotations-api.md)
- [Analyze Build API](/guides/dws-processor/tools-and-api/analyze-build-api.md)
- [Office-to-PDF API](/guides/dws-processor/tools-and-api/office-to-pdf-api.md)
- [Tools and APIs](/guides/dws-processor/tools-and-api.md)
- [Import Instant JSON API](/guides/dws-processor/tools-and-api/import-instant-json-api.md)
- [Markdown-to-PDF API](/guides/dws-processor/tools-and-api/markdown-to-pdf-api.md)
- [PDF flatten API](/guides/dws-processor/tools-and-api/pdf-flatten-api.md)
- [PDF linearization API](/guides/dws-processor/tools-and-api/pdf-linearization-api.md)
- [PDF merge API](/guides/dws-processor/tools-and-api/pdf-merge-api.md)
- [PDF optimization API](/guides/dws-processor/tools-and-api/pdf-optimization-api.md)
- [PDF form filling API](/guides/dws-processor/tools-and-api/pdf-form-filling-api.md)
- [PDF security API](/guides/dws-processor/tools-and-api/pdf-security-api.md)
- [PDF rotate API](/guides/dws-processor/tools-and-api/pdf-rotate-api.md)
- [PDF page manipulation API](/guides/dws-processor/tools-and-api/pdf-page-manipulation-api.md)
- [PDF-to-image API](/guides/dws-processor/tools-and-api/pdf-to-image-api.md)
- [PDF split API](/guides/dws-processor/tools-and-api/pdf-split-api.md)
- [PDF watermark API](/guides/dws-processor/tools-and-api/pdf-watermark-api.md)
- [PDF OCR API](/guides/dws-processor/tools-and-api/pdf-ocr-api.md)
- [PDF-to-HTML API](/guides/dws-processor/tools-and-api/pdf-to-html-api.md)
- [PDF-to-PDF/A API](/guides/dws-processor/tools-and-api/pdf-to-pdfa-api.md)
- [PDF generator API](/guides/dws-processor/tools-and-api/pdf-generator-api.md)
- [PDF/UA auto-tagging API](/guides/dws-processor/tools-and-api/pdfua-api.md)
- [PDF-to-Markdown API](/guides/dws-processor/tools-and-api/pdf-to-markdown-api.md)
- [PDF-to-Office API](/guides/dws-processor/tools-and-api/pdf-to-office-api.md)
- [Redaction API](/guides/dws-processor/tools-and-api/redaction-api.md)