> For the complete documentation index, see [llms.txt](https://docs.apryse.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.apryse.com/xamarin/digital-signature/sign-pdf.md).

# Sign a PDF document in Xamarin

Learn how to digitally sign PDF files with the Apryse SDK. Add approval signatures using a Digital Certificate for authenticity and security. Get a full code sample for signing PDF documents. The Apry

To sign an existing approval signature field in a PDF Document:

{% tabs %}
{% tab title="C#" %}
{% code lineNumbers="true" %}

```csharp
using (PDFDoc doc = new PDFDoc(docpath))
{
  // Retrieve the unsigned approval signature field.
  Field found_approval_field = doc.GetField(approval_field_name);
  DigitalSignatureField approval_digsig_field = new DigitalSignatureField(found_approval_field);

  // (OPTIONAL) Add more information to the signature dictionary.
  approval_digsig_field.SetLocation("Vancouver, BC");
  approval_digsig_field.SetReason("Document approval.");
  approval_digsig_field.SetContactInfo("www.apryse.com");

  // (OPTIONAL) Add an appearance to the signature field.
  Image img = Image.Create(doc, appearance_img_path);
  SignatureWidget approval_signature_widget = new SignatureWidget(found_approval_field.GetSDFObj());
  approval_signature_widget.CreateSignatureAppearance(img);

  // Prepare the signature and signature handler for signing.
  approval_digsig_field.SignOnNextSave(private_key_file_path, keyfile_password);

  // The actual approval signing will be done during the following incremental save operation.
  doc.Save(outpath, SDFDoc.SaveOptions.e_incremental);
}
```

{% endcode %}
{% endtab %}
{% endtabs %}

[Digitally sign PDF files](/xamarin/get-started/samples.md#digitalsignatures) Full code sample which demonstrates using the digital signature API to digitally sign and/or certify PDF documents.

## About Adding An Approval Signature to a PDF Document

The Apryse SDK enables approval signatures in PDF documents using a Digital Certificate, in accordance with the latest PDF specification. By leveraging [public key infrastructure (PKI)](https://en.wikipedia.org/wiki/Public_key_infrastructure) technology, with a certificate issued by a trusted [certificate authority (CA)](https://en.wikipedia.org/wiki/Certificate_authority), a signer can use a certificate-based digital ID to guarantee the authenticity of a signature. Placement of a digital signature using a certificate can also guarantee that a document was not modified since the signature was placed, ensuring the authenticity of the document.

![](https://653871032-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FgjsBtuYmcKhWOdM9VCg4%2Fuploads%2Fgit-blob-51c4b698c1ec2efcc0d46686d1342347626f1a9d%2F8130e2be0155c3a56bbf8548f836090a8beb0ac2-299x658.png?alt=media)

Image taken from Apryse WebViewer

Above is an example of a document containing a certified signature, guaranteed by a certificate generated by Apryse.com.


---

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