using IronPdf; // Disable local disk access or cross-origin requests Installation.EnableWebSecurity = true; // Instantiate Renderer var renderer = new ChromePdfRenderer(); // Create a PDF from a HTML string using C# var pdf = renderer.RenderHtmlAsPdf("<h1>Hello World</h1>"); // Export to a file or Stream pdf.SaveAs("output.pdf"); // Advanced Example with HTML Assets // Load external html assets: Images, CSS and JavaScript. // An optional BasePath 'C:\site\assets\' is set as the file location to load assets from var myAdvancedPdf = renderer.RenderHtmlAsPdf("<img src='icons/iron.png'>", @"C:\site\assets\"); myAdvancedPdf.SaveAs("html-with-assets.pdf");

PDF 工具

如何免费为 PDF 添加密码保护

Curtis Chau

已更新:六月 22, 2025

In today's digital world, ensuring the security of your sensitive data is of utmost importance. Whether it's personal information, financial records, or confidential reports, you may want to restrict permissions and access to your PDF files by adding a password. Fortunately, you don't need to invest in expensive software to add a password, as there are many free version PDF tools available other than Adobe Reader.

In this article, we'll show you how to password protect a PDF for free using three security methods: online services Sejda and SmallPDF, and IronPDF - a C# PDF library.

Sejda - Online PDF Tool

Sejda is a popular online PDF editor that offers a range of features, including password protection. Here's how to use Sejda to protect your PDF:

Step 1: Visit the Sejda Website

Open your web browser and visit the Sejda PDF Editor website.

Step 2: Choose "Protect PDF"

On the Sejda homepage, find and navigate to the Sejda Protect PDF Tool.

How to Password Protect a PDF for Free: Figure 1

Step 3: Upload Your PDF

Click the "Upload PDF file" button to upload the PDF file you want to protect.

How to Password Protect a PDF for Free: Figure 2

Step 4: Set a Password

After uploading the file, you'll have the option to set a password. Set a strong password to limit access to authorized users only.

How to Password Protect a PDF for Free: Figure 3

Step 5: Save the Protected PDF

Once you've set the password, click the "Encrypt PDF" button. Sejda will process the file and provide you with a download link for the password-protected PDF. Click on the download link to save the successfully protected PDF to your computer.

How to Password Protect a PDF for Free: Figure 4

Smallpdf - Online PDF Tool

Smallpdf is a popular online PDF editing tool that offers a range of features, including password protection. Here's how to use Smallpdf to protect your PDF:

Step 1: Go to the Smallpdf Website

Open your web browser and visit the Smallpdf Online PDF Tools website.

Step 2: Access the "Protect PDF" Tool

On the Smallpdf homepage, find and navigate to the Smallpdf Protect PDF Feature.

How to Password Protect a PDF for Free: Figure 5

Step 3: Choose Your PDF Document

Click the "Choose file" button to upload the PDF file you want to protect using a password.

How to Password Protect a PDF for Free: Figure 6

Step 4: Set a Password

After uploading the file, you'll have the option to set a password. Enter a secure password to gain access to only authorized users.

How to Password Protect a PDF for Free: Figure 7

Step 5: Save the Protected PDF Document

Once you've set the password, click the "Encrypt PDF" button. Smallpdf will process the file and provide you with a download link for the encrypted file. Click on the download link to save the protected PDF to your computer.

How to Password Protect a PDF for Free: Figure 8

IronPDF 简介

While both Sejda and Smallpdf are excellent options for password-protecting PDF files with the AES encryption algorithm, they may not be suitable for everyone. Both rely on an internet connection. If you're looking for a more versatile solution with advanced features, you might consider IronPDF.

IronPDF is a robust PDF editing tool equipped with an array of functionalities, including the capability to secure PDF files with passwords. It seamlessly operates as a standalone desktop application or can be seamlessly integrated into your web application through the IronPDF API. Moreover, IronPDF extends its compatibility to both Windows and Mac operating systems.

IronPDF allows you to encrypt and decrypt PDF documents by setting permissions and password protection on the resulting PDF documents, ensuring document security when necessary.

Password Protect a PDF Using IronPDF

To password-protect a PDF using IronPDF, follow these steps:

Step 1: Install IronPDF

Obtain and set up IronPDF by visiting the IronPDF Official Website. Alternatively, you can swiftly install it through the NuGet Package Manager within Visual Studio.

Step 2: Import IronPDF and Open Your PDF

Open the PDF file you want to protect using IronPDF.

using IronPdf;

// Open an existing PDF file or create a new PDF from HTML
var pdf = PdfDocument.FromFile("input.pdf");

using IronPdf;

// Open an existing PDF file or create a new PDF from HTML
var pdf = PdfDocument.FromFile("input.pdf");

Imports IronPdf

' Open an existing PDF file or create a new PDF from HTML
Private pdf = PdfDocument.FromFile("input.pdf")

$vbLabelText $csharpLabel

In the code above, we import the IronPdf namespace and create an instance of the PdfDocument class by loading the specified PDF file named "input.pdf". You can replace this with the path to your PDF file.

Step 3: Set a Password

IronPDF allows you to set a user password for the PDF document to open along with other advanced security settings.

pdf.SecuritySettings.UserPassword = "open123"; // Set a user password for the PDF document

pdf.SecuritySettings.UserPassword = "open123"; // Set a user password for the PDF document

pdf.SecuritySettings.UserPassword = "open123" ' Set a user password for the PDF document

$vbLabelText $csharpLabel

In this line, we're setting a user password open123 to restrict access to authorized users only. It's crucial to use a strong password following best practices for password security.

IronPDF also provides an OwnerPassword option to allow or restrict editing of the PDF document.

Step 4: Save the Protected PDF

Save the PDF with the password protection in place with the SaveAs method.

pdf.SaveAs("secured.pdf");

pdf.SaveAs("secured.pdf");

pdf.SaveAs("secured.pdf")

$vbLabelText $csharpLabel

This command saves the PDF document with the applied security settings to a new file called "secured.pdf". Ensure you provide an appropriate file path as needed.

Your PDF is now secured. Users will need the password to open the document.

How to Password Protect a PDF for Free: Figure 9

You can even confirm password protection by viewing document properties. IronPDF provides a 128-bit RC4 encryption for optimal security.

How to Password Protect a PDF for Free: Figure 10

结论

Protecting your PDF documents with passwords is a simple yet effective way to safeguard your sensitive information. Whether you prefer using the Protect tool in Sejda, another online tool like Smallpdf, or a more versatile solution with multiple security options like IronPDF, all methods allow you to add an extra layer of security to your PDF files without any cost. By following the steps outlined in this article, you can keep your documents safe from unauthorized access and ensure your data remains confidential.

IronPDF offers stronger security compared to basic password protection like online tools do. It includes features like encryption, document permissions password, and digital signatures, providing enhanced protection against unauthorized access and alterations. All this without the need for Adobe Acrobat Reader. Its flexibility makes it easy to integrate into different systems, making it a versatile choice for safeguarding PDFs.

To incorporate IronPDF into your project, you can acquire a commercial license, which begins at $799 for a single developer. Additionally, there's a free trial with complete features and support, allowing you to evaluate its suitability before making a purchase. For those in need of licenses for multiple developers or larger enterprises, various options are accessible.