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Published July 12, 2023

How to Sign A PDF File in Node.JS

Sign PDF documents with ease using Node.js. The ability to digitally sign PDFs not only ensures their authenticity but also enhances their security and reliability. With Node.js, developers can leverage powerful libraries and frameworks to implement seamless digital signing functionality. By utilizing cryptographic algorithms and digital certificates, Node.js enables the creation of digital signatures that are unique to each signer. Whether you're building a document management system, an e-commerce platform, or any application that requires secure document verification, Node.js provides the tools and flexibility to efficiently sign PDFs, empowering you to protect your data and maintain the integrity of your documents.

For signing PDF documents we will use an open source library named PDF-Lib for Node.JS.

1. PDF-Lib

PDF-lib is a popular JavaScript library that provides a simple and powerful way to programmatically create, modify, and extract content from PDF documents. It enables developers to perform various operations on PDFs, such as adding text, images, and annotations, merging and splitting documents, extracting text and images, and digitally signing PDFs. PDF-lib is designed to work seamlessly in both browser-based and server-side environments, making it a versatile tool for manipulating PDF files. With its intuitive API and extensive functionality, PDF-lib simplifies working with PDFs and empowers developers to automate PDF-related tasks efficiently and effectively.

2. Setting up the Environment

Before getting started with digital signature make sure your environment is set.

1. First JavaScript should be installed on your system, if not you can download it from this link.
2. Make sure you have a compatible compiler or text editor on your system, like VS Code.
3. No need to create a whole project just create a JS file and open it in VS Code.

4. You have to install PDF-Lib on your system. You can easily do that by running the following command from the terminal.

   npm install pdf-lib

   npm install pdf-lib

   NODE.JS

Once all the requirements are set you can start development.

3. Sign PDF documents using Node.JS

Open the JavaScript file in VS Code or any text editor. First we will import the PDF-Lib and FS.

const { PDFDocument, StandardFonts, rgb } = require('pdf-lib');
const fs = require('fs');

const { PDFDocument, StandardFonts, rgb } = require('pdf-lib');
const fs = require('fs');

Now we will create a method SignPDF that will take inputPath, signatureText, outputPath as input parameters and create a PDF document as output.

async function signPDF(inputPath, signatureText, outputPath) {
    // Read the PDF document
    const pdfBytes = fs.readFileSync(inputPath);

    // Load the document
    const pdfDoc = await PDFDocument.load(pdfBytes);

    // Embed the signature font
    const signatureFont = await pdfDoc.embedFont(StandardFonts.Helvetica);

    // Get the first page of the PDF
    const pages = pdfDoc.getPages();
    const firstPage = pages[0];

    // Add the signature text as an overlay
    firstPage.drawText(signatureText, {
    x: 50,
    y: 50,
    size: 12,
    font: signatureFont,
    color: rgb(0, 0, 0),
    });

    // Save the modified PDF to a file
    const modifiedPdfBytes = await pdfDoc.save();
    fs.writeFileSync(outputPath, modifiedPdfBytes);

    console.log('PDF signed successfully!');
}

async function signPDF(inputPath, signatureText, outputPath) {
    // Read the PDF document
    const pdfBytes = fs.readFileSync(inputPath);

    // Load the document
    const pdfDoc = await PDFDocument.load(pdfBytes);

    // Embed the signature font
    const signatureFont = await pdfDoc.embedFont(StandardFonts.Helvetica);

    // Get the first page of the PDF
    const pages = pdfDoc.getPages();
    const firstPage = pages[0];

    // Add the signature text as an overlay
    firstPage.drawText(signatureText, {
    x: 50,
    y: 50,
    size: 12,
    font: signatureFont,
    color: rgb(0, 0, 0),
    });

    // Save the modified PDF to a file
    const modifiedPdfBytes = await pdfDoc.save();
    fs.writeFileSync(outputPath, modifiedPdfBytes);

    console.log('PDF signed successfully!');
}

The above code is heavily commented on for you to understand its functionality.

Now our method is written we need to call it and pass the required parameters. First we will define those parameters and then call the method here as in the following example:

// Usage example
const inputPath = 'Print.pdf'; // Path to the input PDF file
const signatureText = 'Signed by DrowzyOzil'; // Signature text to be displayed
const outputPath = 'signed.pdf'; // Path to save the signed PDF

signPDF(inputPath, signatureText, outputPath);

// Usage example
const inputPath = 'Print.pdf'; // Path to the input PDF file
const signatureText = 'Signed by DrowzyOzil'; // Signature text to be displayed
const outputPath = 'signed.pdf'; // Path to save the signed PDF

signPDF(inputPath, signatureText, outputPath);

This is all the code we need to sign PDF documents using JS.

Before we run the code we need a PDF file to Sign for that purpose I will take an example PDF file. Make sure you change the Input path as you need.

3.1. Input PDF file

Once the path is set, let's run the code to get a signed PDF file. You can do this in multiple ways first from terminal and second from VS Code.

3.2. Running JS File from Terminal

To run this code from the terminal just go to the location where the file is located and open the terminal on that location.

In the terminal write the command like this and your Signed PDF file will be generated.

node .\signature.js 

node .\signature.js 

3.3. Output PDF document

Just like that PDF is signed successfully, you can easily change the signature length and content and also its formatting like making the signature transparent, changing its location and many more.

4.1. IronPDF the Best PDF Library to Sign PDF documents

IronPDF is a comprehensive C# library that provides powerful PDF generation, manipulation, and signing capabilities. It allows developers to create, modify, and sign PDF documents with ease. One of the standout features of IronPDF is its ability to sign PDF documents digitally. Digital signatures provide a secure and tamper-evident way to authenticate the integrity and origin of a document. IronPDF supports the creation and application of digital signatures using industry-standard cryptographic algorithms. It allows developers to generate digital certificates, sign PDFs with private keys, and embed signature information within the document. This ensures that the signed PDFs are legally binding and maintain their integrity throughout their lifecycle.

IronPDF offers a range of advanced features that make it the preferred choice for PDF signing. Here are some reasons why it stands out as the best PDF library for signing PDF documents:

Ease of Use

IronPDF provides a simple and intuitive API that makes it easy to add digital signatures to PDFs. The library abstracts away the complexities of cryptographic operations, allowing developers to focus on the core signing functionality.

Extensive Documentation and Support

IronPDF offers comprehensive documentation, tutorials, and examples that guide developers through the process of PDF signing. In addition, the library has an active support community that can assist with any questions or issues that may arise during implementation.

Cross-platform Compatibility

IronPDF is designed to work seamlessly across different platforms and environments. It supports Windows, macOS, and Linux, making it suitable for a wide range of applications.

Security and Compliance

IronPDF adheres to industry standards and best practices for PDF signing, ensuring the highest level of security and compliance.

Extensibility

IronPDF provides a flexible and extensible architecture that allows developers to customize and extend the PDF signing functionality to meet specific requirements.

4.2. Installing IronPDF

There are many ways to install IronPDF, but we will discuss how to install IronPDF using NuGet package manager in Visual Studio.

1. In Visual Studio go to Tools, then hover on NuGet package manager and select Manage NuGet Packages for Solution.

2. It will open a new tab, in this new tab click on browse and search "IronPDF". A list will appear and from that list select the latest version of IronPDF and click the install button.

4.3. Sign PDF using IronPDF

Using IronPDF you can easily create a Digital signature with just a few lines of code.

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");

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");

4.3.1. Output

If you need a detailed tutorial on Signing PDF programmatically, visit here.

5. Conclusion

Signing a PDF document is a critical aspect of document integrity and security, and developers have access to powerful tools like PDF-lib or IronPDF for C# to accomplish this task.

PDF-lib simplifies PDF manipulation with its comprehensive features, allowing developers to load, modify, and save PDFs while adding text overlays or images as signatures. On the other hand, IronPDF excels in PDF generation, manipulation, and PDF Signature capabilities, offering a user-friendly API and cross-platform compatibility.

IronPDF provides a 30-day-free-trial for commercial use to test out its complete functionality. You can download the software from here.

To know more about IronPDF and its features visit the following link.