Digitally Sign a PDF Document

Chaknith Bin

January 25, 2023

Updated December 24, 2024

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Important Disambiguation: Signing a PDF

Developers commonly ask how they can programmatically add a signature to a PDF document using IronPDF. Commonly, signing means different things to different developers:

To digitally sign a PDF document with a Certificate to ensure it cannot be tampered with.
To add a graphical handwritten signature image to an existing PDF from an image file.
To stamp a Certificate's Image onto a PDF.
To add a Signature Form Field to a PDF which some viewers can prompt for Signing.

Get started with IronPDF

Start using IronPDF in your project today with a free trial.

First Step:

How to Sign PDF File in C#

Install the IronPDF library to sign PDF documents
Add granular detail and sign PDF from X509Certificate2
Add visual image to represent the digital signature in many ways
Stamp a handwritten signature onto PDF file
Add a signature form field in C#

Sign a PDF with a Digital Certificate

IronPDF supports many ways to sign a PDF with a digital signature certificate of .pfx and .p12 formats. In this How-To guide, you will be guided through all three main methods used to digitally sign PDF documents:

Signing Method	Description
Sign	Sign a PDF with a PdfSignature object
SignWithFile	Sign PDF with a digital signature certificate(.pfx or .p12) on disk
SignWithStore	Signs the PDF with digital signature extracted from your computer's signature storage. Based on a thumbprint ID

Supported Digital Signature Certificate Files

We officially comply with the X509Certificate2 standard and support .pfx and .p12 signatures. If your signature is unable to be applied directly in IronPDF's signing methods, you will need to create a X509Certificate2 certificate with instructions that can be found on the Microsoft documentation.

Sign: Create PdfSignature From X509Certificate2

IronPDF's Signing methods will accept X509Certificate2 objects with X509KeyStorageFlags set to Exportable.

Please note

IronPDF only supports X509KeyStorageFlags.Exportable. Some certificates have KeyStorageFlags set to Exportable by default. Attempting to use different KeyStorageFlags will result in an exception => Internal.Cryptography.CryptoThrowHelper.WindowsCryptographicException: 'The requested operation is not supported.'

:path=/static-assets/pdf/content-code-examples/how-to/signing-X509Certificate2-with-privatekey.cs

using IronPdf;
using IronPdf.Signing;
using System.Security.Cryptography.X509Certificates;

ChromePdfRenderer renderer = new ChromePdfRenderer();
PdfDocument pdf = renderer.RenderHtmlAsPdf("<h1>foo</h1>");

// Create X509Certificate2 object with X509KeyStorageFlags set to Exportable
X509Certificate2 cert = new X509Certificate2("IronSoftware.pfx", "123456", X509KeyStorageFlags.Exportable);

// Create PdfSignature object
var sig = new PdfSignature(cert);

// Sign PDF document
pdf.Sign(sig);

pdf.SaveAs("signed.pdf");

Imports IronPdf
Imports IronPdf.Signing
Imports System.Security.Cryptography.X509Certificates

Private renderer As New ChromePdfRenderer()
Private pdf As PdfDocument = renderer.RenderHtmlAsPdf("<h1>foo</h1>")

' Create X509Certificate2 object with X509KeyStorageFlags set to Exportable
Private cert As New X509Certificate2("IronSoftware.pfx", "123456", X509KeyStorageFlags.Exportable)

' Create PdfSignature object
Private sig = New PdfSignature(cert)

' Sign PDF document
pdf.Sign(sig)

pdf.SaveAs("signed.pdf")

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Sign: Add Granular Detail to PdfSignature

While instantiating PdfSignature or after it's instantiated, useful information can be added to the PdfSignature object including date, signing contact, location, signing reason, timestamp, and adding image as a visual appearance on the PDF document.

Tips

Supports timestamping servers that require SHA256 and SHA512

:path=/static-assets/pdf/content-code-examples/how-to/signing-add-granular-information.cs

using IronPdf;
using IronPdf.Signing;
using IronSoftware.Drawing;
using System;

ChromePdfRenderer renderer = new ChromePdfRenderer();
PdfDocument pdf = renderer.RenderHtmlAsPdf("<h1>foo</h1>");

pdf.SaveAs("signed.pdf");

// Create PdfSignature object
var sig = new PdfSignature("IronSoftware.pfx", "123456");

// Add granular information
sig.SignatureDate = new DateTime(2000, 12, 02);
sig.SigningContact = "IronSoftware";
sig.SigningLocation = "Chicago";
sig.SigningReason = "How to guide";
sig.TimestampHashAlgorithm = TimestampHashAlgorithms.SHA256;
sig.TimeStampUrl = "http://timestamp.digicert.com";
sig.SignatureImage = new PdfSignatureImage("IronSoftware.png", 0, new Rectangle(0, 600, 100, 100));

// Sign and save PDF document
sig.SignPdfFile("signed.pdf");

Imports IronPdf
Imports IronPdf.Signing
Imports IronSoftware.Drawing
Imports System

Private renderer As New ChromePdfRenderer()
Private pdf As PdfDocument = renderer.RenderHtmlAsPdf("<h1>foo</h1>")

pdf.SaveAs("signed.pdf")

' Create PdfSignature object
Dim sig = New PdfSignature("IronSoftware.pfx", "123456")

' Add granular information
sig.SignatureDate = New DateTime(2000, 12, 02)
sig.SigningContact = "IronSoftware"
sig.SigningLocation = "Chicago"
sig.SigningReason = "How to guide"
sig.TimestampHashAlgorithm = TimestampHashAlgorithms.SHA256
sig.TimeStampUrl = "http://timestamp.digicert.com"
sig.SignatureImage = New PdfSignatureImage("IronSoftware.png", 0, New Rectangle(0, 600, 100, 100))

' Sign and save PDF document
sig.SignPdfFile("signed.pdf")

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Demonstration

You might see an exclamation mark or warning sign instead of a check mark. This happens because Adobe cannot confirm the authenticity and integrity of the document since the certificate is not present. To get the check mark, please add the certificate to Adobe and reopen the document.

Different Ways to Add Image

Image can be included in many ways:

Set SignatureImage property to a new PdfSignatureImage object.
Use LoadSignatureImageFromFile method to load image from a file. The LoadSignatureImageFromFile supports various image formats.
Use LoadSignatureImageFromStream method to load image from stream. The image stream can be generated from other libraries as long as it has the formats of TGA, PBM, TIFF, BMP, GIF, PNG, JPEG, Webp.

:path=/static-assets/pdf/content-code-examples/how-to/signing-add-image.cs

using IronPdf.Signing;
using IronSoftware.Drawing;

// Create PdfSignature object
var sig = new PdfSignature("IronSoftware.pfx", "123456");

// Add image by property
sig.SignatureImage = new PdfSignatureImage("IronSoftware.png", 0, new Rectangle(0, 600, 100, 100));

// Add image by LoadSignatureImageFromFile method
sig.LoadSignatureImageFromFile("IronSoftware.png", 0, new Rectangle(0, 600, 100, 100));

// Import image using IronSoftware.Drawing
AnyBitmap image = AnyBitmap.FromFile("IronSoftware.png");

sig.LoadSignatureImageFromStream(image.ToStream(), 0, new Rectangle(0, 600, 100, 100));

Imports IronPdf.Signing
Imports IronSoftware.Drawing

' Create PdfSignature object
Private sig = New PdfSignature("IronSoftware.pfx", "123456")

' Add image by property
sig.SignatureImage = New PdfSignatureImage("IronSoftware.png", 0, New Rectangle(0, 600, 100, 100))

' Add image by LoadSignatureImageFromFile method
sig.LoadSignatureImageFromFile("IronSoftware.png", 0, New Rectangle(0, 600, 100, 100))

' Import image using IronSoftware.Drawing
Dim image As AnyBitmap = AnyBitmap.FromFile("IronSoftware.png")

sig.LoadSignatureImageFromStream(image.ToStream(), 0, New Rectangle(0, 600, 100, 100))

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Signature Permissions

You may explicitly specify conditions under which your certificate will remain valid. If you want your signature invalidated upon any change, or allowing just form field changes, etc., please use the following table to see the options:

PdfDocument.SignaturePermissions	Definition
NoChangesAllowed	No changes are allowed
FormFillingAllowed	Changing form field values allowed
FormFillingAndAnnotationsAllowed	Changing form field values and modifying annotations allowed

This parameter is optional and not setting it will apply a signature that certifies a specific revision and cannot be invalidated.

Save and Sign a PDF Revision Iteration

In the following example, we open a PDF file, make various edits, then before we save, we will sign it. For signature permissions, we will only allow form-filling as future edits; otherwise, the signature will be invalidated by any other edit.

We will then call SaveAsRevision to save the revision to the history and then save our new document to disk.

:path=/static-assets/pdf/content-code-examples/how-to/signing-revision.cs

using IronPdf;
using IronPdf.Rendering;

// Import PDF and enable TrackChanges
PdfDocument pdf = PdfDocument.FromFile("annual_census.pdf", TrackChanges: ChangeTrackingModes.EnableChangeTracking);
// ... various edits ...
pdf.SignWithFile("/assets/IronSignature.p12", "password", null, IronPdf.Signing.SignaturePermissions.AdditionalSignaturesAndFormFillingAllowed);

PdfDocument pdfWithRevision = pdf.SaveAsRevision();

pdfWithRevision.SaveAs("annual_census_2.pdf");

Imports IronPdf
Imports IronPdf.Rendering

' Import PDF and enable TrackChanges
Private pdf As PdfDocument = PdfDocument.FromFile("annual_census.pdf", TrackChanges:= ChangeTrackingModes.EnableChangeTracking)
' ... various edits ...
pdf.SignWithFile("/assets/IronSignature.p12", "password", Nothing, IronPdf.Signing.SignaturePermissions.AdditionalSignaturesAndFormFillingAllowed)

Dim pdfWithRevision As PdfDocument = pdf.SaveAsRevision()

pdfWithRevision.SaveAs("annual_census_2.pdf")

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Understanding Incremental Saving for Signatures

While some viewers like Chrome browsers only show one version, PDF files have the capability to store previous versions of the document similar to a Git commit history. You will see this in more advanced PDF viewers such as Adobe Acrobat.

When dealing with PDF signatures, it is important to know about this because the action of signing a PDF applies to the current iteration of the PDF. Your PDF may have signatures for older iterations, or may have a few unsigned versions. We can visualize an example as follows:

PDF Document Iteration	Certificate A	Certificate B	Certificate C	Certificate D
0 (first save)	✅
1
2
3	✅ (form field edits only)	✅ (form field edits only)
4 (only form fields edited)			✅
5	✅ (no further edits allowed)	✅ (no further edits allowed)		✅ (no further edits allowed)

Above, we have documents that have been through 6 different iterations. This document may be being passed around departments of a company with approval until being finalized at iteration 3. In this iteration, both Person A and Person B signed the document with the permission "Form Field Edits Only" set. This means that filling in form fields in the PDF document is allowed, but any other change to the document will invalidate their signatures.

In the example above, we can assume Person C is the one who has filled out the form and sent it back to Person A, B, and D who all signed the document a final time with the "No Edits Allowed" permission. Since no invalidating actions were taken in this document, when we run IronPDF's signature method, we will get true.

Roll Back to an Old Revision

To roll back to a previous revision of a PDF, you can use the GetRevision method. This will forget any changes made since this revision including newer signatures. To do this use:

:path=/static-assets/pdf/content-code-examples/how-to/signing-revert-revision.cs

using IronPdf;

PdfDocument pdf = PdfDocument.FromFile("report.pdf");

int versions = pdf.RevisionCount; // total revisions

PdfDocument rolledBackPdf = pdf.GetRevision(2);
rolledBackPdf.SaveAs("report-draft.pdf");

Imports IronPdf

Private pdf As PdfDocument = PdfDocument.FromFile("report.pdf")

Private versions As Integer = pdf.RevisionCount ' total revisions

Private rolledBackPdf As PdfDocument = pdf.GetRevision(2)
rolledBackPdf.SaveAs("report-draft.pdf")

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Remove Signatures

IronPDF has a method RemoveSignatures that will remove every signature of every revision for a PDF document. Usage is as follows:

:path=/static-assets/pdf/content-code-examples/how-to/signing-remove-signature.cs

using IronPdf;

PdfDocument pdf = PdfDocument.FromFile("invoice.pdf");
pdf.RemoveSignatures();

Imports IronPdf

Private pdf As PdfDocument = PdfDocument.FromFile("invoice.pdf")
pdf.RemoveSignatures()

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Verify All Signatures in a PDF

Calling the verify signatures method on a PDF document will look at all Signatures from all document iterations and verify that all of them are still valid. This will return a bool of true if they are all valid.

:path=/static-assets/pdf/content-code-examples/how-to/signing-verify-signatures.cs

using IronPdf;

PdfDocument pdf = PdfDocument.FromFile("annual_census.pdf");
bool isValid = pdf.VerifyPdfSignatures();

Imports IronPdf

Private pdf As PdfDocument = PdfDocument.FromFile("annual_census.pdf")
Private isValid As Boolean = pdf.VerifyPdfSignatures()

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Stamp a Signature onto a PDF

First off, I will start with a PDF I want to sign. I will use this example invoice:

We will apply a handwritten signature which is in the form of a .png image to our PDF. This may be a handwritten signature or the image that was used in the creation of a certificate file. This is the sample signature we will use:

Code

With this, we will use the following code to stamp the handwritten signature to the PDF as a watermark:

:path=/static-assets/pdf/content-code-examples/how-to/signing-handwritten.cs

using IronPdf;
using IronPdf.Editing;

var pdf = PdfDocument.FromFile("invoice.pdf");

pdf.ApplyWatermark("<img src='signature.png'/>", 90, VerticalAlignment.Bottom, HorizontalAlignment.Right);

pdf.SaveAs("official_invoice.pdf");

Imports IronPdf
Imports IronPdf.Editing

Private pdf = PdfDocument.FromFile("invoice.pdf")

pdf.ApplyWatermark("<img src='signature.png'/>", 90, VerticalAlignment.Bottom, HorizontalAlignment.Right)

pdf.SaveAs("official_invoice.pdf")

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Output Result

After this code is run, we have this output file which has our signature on the bottom right:

Add an Unsigned Signature Field to a PDF

To add an unsigned or empty signature form field, you must first create the signature form field by instantiating the signature object. Next, on the target PDF document, access the Form property and pass the newly created signature object to the Add method. Finally, export the PDF with the empty signature form.

:path=/static-assets/pdf/content-code-examples/how-to/signing-unsigned-signature.cs

using IronPdf;
using IronSoftware.Forms;

ChromePdfRenderer renderer = new ChromePdfRenderer();
PdfDocument pdf = renderer.RenderHtmlAsPdf("<h1>testing</h1>");

// Configure required parameters
string name = "cert";
uint pageIndex = 0;
double x = 100;
double y = 600;
double width = 300;
double height = 100;

// Create signature
SignatureFormField signature = new SignatureFormField(name, pageIndex, x, y, width, height);

// Add signature
pdf.Form.Add(signature);

pdf.SaveAs("signature.pdf");

Imports IronPdf
Imports IronSoftware.Forms

Private renderer As New ChromePdfRenderer()
Private pdf As PdfDocument = renderer.RenderHtmlAsPdf("<h1>testing</h1>")

' Configure required parameters
Private name As String = "cert"
Private pageIndex As UInteger = 0
Private x As Double = 100
Private y As Double = 600
Private width As Double = 300
Private height As Double = 100

' Create signature
Private signature As New SignatureFormField(name, pageIndex, x, y, width, height)

' Add signature
pdf.Form.Add(signature)

pdf.SaveAs("signature.pdf")

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You can read more about IronPDF and the forms it supports in the How to Create PDF Forms article.

Chaknith Bin

Chat with engineering team now

Software Engineer

Chaknith works on IronXL and IronBarcode. He has deep expertise in C# and .NET, helping improve the software and support customers. His insights from user interactions contribute to better products, documentation, and overall experience.