How to Compare Two PDF Files in C#

Prerequisites and Setup

Before starting, ensure you have installed:

• Visual Studio 2019 or later
• .NET Framework 4.6.2+ or .NET Core 3.1+
• Basic C# knowledge

Install-Package IronPdf

Install-Package IronPdf

dotnet add package IronPdf

dotnet add package IronPdf

using IronPdf;
using System;

using IronPdf;
using System;

Basic PDF Document Comparison

Let's start with sample code to compare PDF files by extracting and comparing their text content:

public class BasicPdfComparer
{
    public static bool ComparePdfFiles(string firstPdfPath, string secondPdfPath)
    {
        // Load two PDF documents
        var pdf1 = PdfDocument.FromFile(firstPdfPath);
        var pdf2 = PdfDocument.FromFile(secondPdfPath);
        // Extract all text from both PDFs
        string text1 = pdf1.ExtractAllText();
        string text2 = pdf2.ExtractAllText();
        // Compare the two documents
        bool areIdentical = text1 == text2;
        // Find differences and calculate similarity
        double similarity = CalculateSimilarity(text1, text2);
        Console.WriteLine($"Documents are {(areIdentical ? "identical" : "different")}");
        Console.WriteLine($"Similarity: {similarity:F2}%");
        return areIdentical;
    }
    private static double CalculateSimilarity(string text1, string text2)
    {
        if (text1 == text2) return 100.0;
        if (string.IsNullOrEmpty(text1) || string.IsNullOrEmpty(text2)) return 0.0;
        int matchingChars = 0;
        int minLength = Math.Min(text1.Length, text2.Length);
        for (int i = 0; i < minLength; i++)
        {
            if (text1[i] == text2[i]) matchingChars++;
        }
        return (double)matchingChars / Math.Max(text1.Length, text2.Length) * 100;
    }
}

public class BasicPdfComparer
{
    public static bool ComparePdfFiles(string firstPdfPath, string secondPdfPath)
    {
        // Load two PDF documents
        var pdf1 = PdfDocument.FromFile(firstPdfPath);
        var pdf2 = PdfDocument.FromFile(secondPdfPath);
        // Extract all text from both PDFs
        string text1 = pdf1.ExtractAllText();
        string text2 = pdf2.ExtractAllText();
        // Compare the two documents
        bool areIdentical = text1 == text2;
        // Find differences and calculate similarity
        double similarity = CalculateSimilarity(text1, text2);
        Console.WriteLine($"Documents are {(areIdentical ? "identical" : "different")}");
        Console.WriteLine($"Similarity: {similarity:F2}%");
        return areIdentical;
    }
    private static double CalculateSimilarity(string text1, string text2)
    {
        if (text1 == text2) return 100.0;
        if (string.IsNullOrEmpty(text1) || string.IsNullOrEmpty(text2)) return 0.0;
        int matchingChars = 0;
        int minLength = Math.Min(text1.Length, text2.Length);
        for (int i = 0; i < minLength; i++)
        {
            if (text1[i] == text2[i]) matchingChars++;
        }
        return (double)matchingChars / Math.Max(text1.Length, text2.Length) * 100;
    }
}

This code loads two PDF files using IronPDF's PdfDocument.FromFile() method, extracts all text content, and performs a direct string comparison. The compare method gives you metrics to identify how closely the documents match.

Page-by-Page Comparison Method

For more detailed analysis, compare two PDFs page by page to identify exactly where differences occur:

public static void ComparePageByPage(string firstPdfPath, string secondPdfPath)
{
    // Load two files
    var pdf1 = PdfDocument.FromFile(firstPdfPath);
    var pdf2 = PdfDocument.FromFile(secondPdfPath);
    int maxPages = Math.Max(pdf1.PageCount, pdf2.PageCount);
    int differencesFound = 0;
    for (int i = 0; i < maxPages; i++)
    {
        // Process each page
        string page1Text = i < pdf1.PageCount ? pdf1.ExtractTextFromPage(i) : "";
        string page2Text = i < pdf2.PageCount ? pdf2.ExtractTextFromPage(i) : "";
        if (page1Text != page2Text)
        {
            differencesFound++;
            Console.WriteLine($"Page {i + 1}: Differences detected");
        }
        else
        {
            Console.WriteLine($"Page {i + 1}: Identical");
        }
    }
    Console.WriteLine($"\nSummary: {differencesFound} page(s) with changes");
}

public static void ComparePageByPage(string firstPdfPath, string secondPdfPath)
{
    // Load two files
    var pdf1 = PdfDocument.FromFile(firstPdfPath);
    var pdf2 = PdfDocument.FromFile(secondPdfPath);
    int maxPages = Math.Max(pdf1.PageCount, pdf2.PageCount);
    int differencesFound = 0;
    for (int i = 0; i < maxPages; i++)
    {
        // Process each page
        string page1Text = i < pdf1.PageCount ? pdf1.ExtractTextFromPage(i) : "";
        string page2Text = i < pdf2.PageCount ? pdf2.ExtractTextFromPage(i) : "";
        if (page1Text != page2Text)
        {
            differencesFound++;
            Console.WriteLine($"Page {i + 1}: Differences detected");
        }
        else
        {
            Console.WriteLine($"Page {i + 1}: Identical");
        }
    }
    Console.WriteLine($"\nSummary: {differencesFound} page(s) with changes");
}

The ExtractTextFromPage() method allows targeted extraction from specific pages. This approach helps developers pinpoint discrepancies between different versions of PDF documents.

Using the Comparer Class Pattern

Create a dedicated comparer class to enhance your comparison functionality:

public class PdfComparer
{
    private PdfDocument pdf1;
    private PdfDocument pdf2;
    private bool disposed;
    public PdfComparer(string file1Path, string file2Path)
    {
        // Load the two PDFs
        pdf1 = PdfDocument.FromFile(file1Path);
        pdf2 = PdfDocument.FromFile(file2Path);
    }
    public ComparisonResult Compare()
    {
        var result = new ComparisonResult();
        // Compare PDF documents
        string text1 = pdf1?.ExtractAllText() ?? string.Empty;
        string text2 = pdf2?.ExtractAllText() ?? string.Empty;
        result.AreIdentical = string.Equals(text1, text2, StringComparison.Ordinal);
        result.SimilarityPercent = CalculateSimilarity(text1, text2);
        result.Differences = FindDifferences(text1, text2);
        return result;
    }
    private List<string> FindDifferences(string text1, string text2)
    {
        var differences = new List<string>();
        // Normalize nulls
        text1 ??= string.Empty;
        text2 ??= string.Empty;
        if (string.Equals(text1, text2, StringComparison.Ordinal))
            return differences;
        // Page-aware comparisons aren't possible here because we only have full-text;
        // produce a concise, actionable difference entry:
        int min = Math.Min(text1.Length, text2.Length);
        int firstDiff = -1;
        for (int i = 0; i < min; i++)
        {
            if (text1[i] != text2[i])
            {
                firstDiff = i;
                break;
            }
        }
        if (firstDiff == -1 && text1.Length != text2.Length)
        {
            // No differing character in the overlap, but lengths differ
            firstDiff = min;
        }
        // Create short excerpts around the first difference for context
        int excerptLength = 200;
        string excerpt1 = string.Empty;
        string excerpt2 = string.Empty;
        if (firstDiff >= 0)
        {
            int start1 = Math.Max(0, firstDiff);
            int len1 = Math.Min(excerptLength, Math.Max(0, text1.Length - start1));
            excerpt1 = start1 < text1.Length ? text1.Substring(start1, len1) : string.Empty;
            int start2 = Math.Max(0, firstDiff);
            int len2 = Math.Min(excerptLength, Math.Max(0, text2.Length - start2));
            excerpt2 = start2 < text2.Length ? text2.Substring(start2, len2) : string.Empty;
        }
        // HTML-encode excerpts if they will be embedded into HTML reports
        string safeExcerpt1 = System.Net.WebUtility.HtmlEncode(excerpt1);
        string safeExcerpt2 = System.Net.WebUtility.HtmlEncode(excerpt2);
        double similarity = CalculateSimilarity(text1, text2);
        differences.Add($"Similarity: {similarity:F2}%. Lengths: [{text1.Length}, {text2.Length}]. First difference index: {firstDiff}. Excerpt1: \"{safeExcerpt1}\" Excerpt2: \"{safeExcerpt2}\"");
        return differences;
    }
    public void Close()
    {
        Dispose();
    }
    public void Dispose()
    {
        if (disposed) return;
        disposed = true;
        pdf1?.Dispose();
        pdf2?.Dispose();
    }
}
public class ComparisonResult
{
    // True when extracted text is exactly the same
    public bool AreIdentical { get; set; }
    // Similarity expressed as percent (0..100)
    public double SimilarityPercent { get; set; }
    // Human readable difference entries
    public List<string> Differences { get; set; } = new List<string>();
}

public class PdfComparer
{
    private PdfDocument pdf1;
    private PdfDocument pdf2;
    private bool disposed;
    public PdfComparer(string file1Path, string file2Path)
    {
        // Load the two PDFs
        pdf1 = PdfDocument.FromFile(file1Path);
        pdf2 = PdfDocument.FromFile(file2Path);
    }
    public ComparisonResult Compare()
    {
        var result = new ComparisonResult();
        // Compare PDF documents
        string text1 = pdf1?.ExtractAllText() ?? string.Empty;
        string text2 = pdf2?.ExtractAllText() ?? string.Empty;
        result.AreIdentical = string.Equals(text1, text2, StringComparison.Ordinal);
        result.SimilarityPercent = CalculateSimilarity(text1, text2);
        result.Differences = FindDifferences(text1, text2);
        return result;
    }
    private List<string> FindDifferences(string text1, string text2)
    {
        var differences = new List<string>();
        // Normalize nulls
        text1 ??= string.Empty;
        text2 ??= string.Empty;
        if (string.Equals(text1, text2, StringComparison.Ordinal))
            return differences;
        // Page-aware comparisons aren't possible here because we only have full-text;
        // produce a concise, actionable difference entry:
        int min = Math.Min(text1.Length, text2.Length);
        int firstDiff = -1;
        for (int i = 0; i < min; i++)
        {
            if (text1[i] != text2[i])
            {
                firstDiff = i;
                break;
            }
        }
        if (firstDiff == -1 && text1.Length != text2.Length)
        {
            // No differing character in the overlap, but lengths differ
            firstDiff = min;
        }
        // Create short excerpts around the first difference for context
        int excerptLength = 200;
        string excerpt1 = string.Empty;
        string excerpt2 = string.Empty;
        if (firstDiff >= 0)
        {
            int start1 = Math.Max(0, firstDiff);
            int len1 = Math.Min(excerptLength, Math.Max(0, text1.Length - start1));
            excerpt1 = start1 < text1.Length ? text1.Substring(start1, len1) : string.Empty;
            int start2 = Math.Max(0, firstDiff);
            int len2 = Math.Min(excerptLength, Math.Max(0, text2.Length - start2));
            excerpt2 = start2 < text2.Length ? text2.Substring(start2, len2) : string.Empty;
        }
        // HTML-encode excerpts if they will be embedded into HTML reports
        string safeExcerpt1 = System.Net.WebUtility.HtmlEncode(excerpt1);
        string safeExcerpt2 = System.Net.WebUtility.HtmlEncode(excerpt2);
        double similarity = CalculateSimilarity(text1, text2);
        differences.Add($"Similarity: {similarity:F2}%. Lengths: [{text1.Length}, {text2.Length}]. First difference index: {firstDiff}. Excerpt1: \"{safeExcerpt1}\" Excerpt2: \"{safeExcerpt2}\"");
        return differences;
    }
    public void Close()
    {
        Dispose();
    }
    public void Dispose()
    {
        if (disposed) return;
        disposed = true;
        pdf1?.Dispose();
        pdf2?.Dispose();
    }
}
public class ComparisonResult
{
    // True when extracted text is exactly the same
    public bool AreIdentical { get; set; }
    // Similarity expressed as percent (0..100)
    public double SimilarityPercent { get; set; }
    // Human readable difference entries
    public List<string> Differences { get; set; } = new List<string>();
}

This class structure provides a clean way to compare PDF documents while managing document objects and references properly.

Creating a Visual Comparison Report

Generate an HTML-based report that highlights differences between two documents and save it as a PDF file:

public static void CreateComparisonReport(string pdf1Path, string pdf2Path, string outputPath)
{
    // Load PDFs for comparison
    var pdf1 = PdfDocument.FromFile(pdf1Path);
    var pdf2 = PdfDocument.FromFile(pdf2Path);
    // Build HTML report with details
    string htmlReport = @"
        <html>
        <head>
            <style>
                body { font-family: Arial, sans-serif; margin: 20px; }
                .identical { color: green; }
                .different { color: red; }
                table { width: 100%; border-collapse: collapse; }
                th, td { border: 1px solid #ddd; padding: 8px; }
            </style>
        </head>
        <body>
            <h1>PDF Comparison Report</h1>
            <table>
                <tr><th>Page</th><th>Status</th></tr>";
    // Process each page
    for (int i = 0; i < Math.Max(pdf1.PageCount, pdf2.PageCount); i++)
    {
        string page1 = i < pdf1.PageCount ? pdf1.ExtractTextFromPage(i) : "";
        string page2 = i < pdf2.PageCount ? pdf2.ExtractTextFromPage(i) : "";
        bool identical = page1 == page2;
        string status = identical ? "Identical" : "Different";
        htmlReport += $"<tr><td>Page {i + 1}</td><td class='{status.ToLower()}'>{status}</td></tr>";
    }
    htmlReport += "</table></body></html>";
    // Create and save the report
    var renderer = new ChromePdfRenderer();
    var reportPdf = renderer.RenderHtmlAsPdf(htmlReport);
    reportPdf.SaveAs(outputPath);
    Console.WriteLine($"Report saved to: {outputPath}");
}

public static void CreateComparisonReport(string pdf1Path, string pdf2Path, string outputPath)
{
    // Load PDFs for comparison
    var pdf1 = PdfDocument.FromFile(pdf1Path);
    var pdf2 = PdfDocument.FromFile(pdf2Path);
    // Build HTML report with details
    string htmlReport = @"
        <html>
        <head>
            <style>
                body { font-family: Arial, sans-serif; margin: 20px; }
                .identical { color: green; }
                .different { color: red; }
                table { width: 100%; border-collapse: collapse; }
                th, td { border: 1px solid #ddd; padding: 8px; }
            </style>
        </head>
        <body>
            <h1>PDF Comparison Report</h1>
            <table>
                <tr><th>Page</th><th>Status</th></tr>";
    // Process each page
    for (int i = 0; i < Math.Max(pdf1.PageCount, pdf2.PageCount); i++)
    {
        string page1 = i < pdf1.PageCount ? pdf1.ExtractTextFromPage(i) : "";
        string page2 = i < pdf2.PageCount ? pdf2.ExtractTextFromPage(i) : "";
        bool identical = page1 == page2;
        string status = identical ? "Identical" : "Different";
        htmlReport += $"<tr><td>Page {i + 1}</td><td class='{status.ToLower()}'>{status}</td></tr>";
    }
    htmlReport += "</table></body></html>";
    // Create and save the report
    var renderer = new ChromePdfRenderer();
    var reportPdf = renderer.RenderHtmlAsPdf(htmlReport);
    reportPdf.SaveAs(outputPath);
    Console.WriteLine($"Report saved to: {outputPath}");
}

This code demonstrates how to create a professional PDF document with comparison results. The report is saved to the specified path.

Comparing Multiple PDF Documents

When you need to compare multiple PDF documents simultaneously, use this method:

public static void CompareMultiplePdfs(params string[] pdfPaths)
{
    if (pdfPaths.Length < 2)
    {
        Console.WriteLine("Need at least two files to compare");
        return;
    }
    // Load multiple PDF documents
    var pdfs = new List<PdfDocument>();
    foreach (var path in pdfPaths)
    {
        pdfs.Add(PdfDocument.FromFile(path));
    }
    // Compare all documents
    for (int i = 0; i < pdfs.Count - 1; i++)
    {
        for (int j = i + 1; j < pdfs.Count; j++)
        {
            string text1 = pdfs[i].ExtractAllText();
            string text2 = pdfs[j].ExtractAllText();
            bool identical = text1 == text2;
            Console.WriteLine($"File {i+1} vs File {j+1}: {(identical ? "Identical" : "Different")}");
        }
    }
    // Close all documents
    pdfs.ForEach(pdf => pdf.Dispose());
}

public static void CompareMultiplePdfs(params string[] pdfPaths)
{
    if (pdfPaths.Length < 2)
    {
        Console.WriteLine("Need at least two files to compare");
        return;
    }
    // Load multiple PDF documents
    var pdfs = new List<PdfDocument>();
    foreach (var path in pdfPaths)
    {
        pdfs.Add(PdfDocument.FromFile(path));
    }
    // Compare all documents
    for (int i = 0; i < pdfs.Count - 1; i++)
    {
        for (int j = i + 1; j < pdfs.Count; j++)
        {
            string text1 = pdfs[i].ExtractAllText();
            string text2 = pdfs[j].ExtractAllText();
            bool identical = text1 == text2;
            Console.WriteLine($"File {i+1} vs File {j+1}: {(identical ? "Identical" : "Different")}");
        }
    }
    // Close all documents
    pdfs.ForEach(pdf => pdf.Dispose());
}

This approach allows comparing multiple PDF documents in a single run, useful for batch processing requirements.

Real-World Contract Version Control

Here's a practical example for tracking revisions in legal documents:

public class ContractVersionControl
{
    public static void CompareContractVersions(string originalPath, string revisedPath)
    {
        // System for comparing contract versions
        Console.WriteLine("Contract Version Comparison System");
        var original = PdfDocument.FromFile(originalPath);
        var revised = PdfDocument.FromFile(revisedPath);
        // Extract contract contents
        string originalText = original.ExtractAllText();
        string revisedText = revised.ExtractAllText();
        if (originalText == revisedText)
        {
            Console.WriteLine("No changes detected");
        }
        else
        {
            Console.WriteLine("Changes detected in revised version");
            // Create detailed report
            string reportPath = $"contract_comparison_{DateTime.Now:yyyyMMdd}.pdf";
            CreateComparisonReport(originalPath, revisedPath, reportPath);
            Console.WriteLine($"Report created: {reportPath}");
        }
        // Close documents
        original.Dispose();
        revised.Dispose();
    }
}

public class ContractVersionControl
{
    public static void CompareContractVersions(string originalPath, string revisedPath)
    {
        // System for comparing contract versions
        Console.WriteLine("Contract Version Comparison System");
        var original = PdfDocument.FromFile(originalPath);
        var revised = PdfDocument.FromFile(revisedPath);
        // Extract contract contents
        string originalText = original.ExtractAllText();
        string revisedText = revised.ExtractAllText();
        if (originalText == revisedText)
        {
            Console.WriteLine("No changes detected");
        }
        else
        {
            Console.WriteLine("Changes detected in revised version");
            // Create detailed report
            string reportPath = $"contract_comparison_{DateTime.Now:yyyyMMdd}.pdf";
            CreateComparisonReport(originalPath, revisedPath, reportPath);
            Console.WriteLine($"Report created: {reportPath}");
        }
        // Close documents
        original.Dispose();
        revised.Dispose();
    }
}

This demonstrates how to compare two PDF contract versions and generate reports for review. The system automatically detects changes and creates documentation.

Handling Password-Protected PDFs

IronPDF seamlessly handles encrypted PDF files by passing the password when calling the load method:

public static bool CompareProtectedPdfs(string pdf1Path, string pass1,
                                        string pdf2Path, string pass2)
{
    // Load password-protected two PDFs
    var pdf1 = PdfDocument.FromFile(pdf1Path, pass1);
    var pdf2 = PdfDocument.FromFile(pdf2Path, pass2);
    // Extract and compare text
    string text1 = pdf1.ExtractAllText();
    string text2 = pdf2.ExtractAllText();
    bool result = text1 == text2;
    // Close and dispose
    pdf1.Dispose();
    pdf2.Dispose();
    return result;
}

public static bool CompareProtectedPdfs(string pdf1Path, string pass1,
                                        string pdf2Path, string pass2)
{
    // Load password-protected two PDFs
    var pdf1 = PdfDocument.FromFile(pdf1Path, pass1);
    var pdf2 = PdfDocument.FromFile(pdf2Path, pass2);
    // Extract and compare text
    string text1 = pdf1.ExtractAllText();
    string text2 = pdf2.ExtractAllText();
    bool result = text1 == text2;
    // Close and dispose
    pdf1.Dispose();
    pdf2.Dispose();
    return result;
}

Simply pass the password parameter when loading protected PDFs. IronPDF handles decryption automatically.

Step-by-Step Instructions

Follow these steps to implement PDF comparison in your .NET project:

1. Install the IronPDF for .NET package via NuGet
2. Add the IronPDF namespace reference
3. Create an instance of PdfDocument for each file
4. Use methods like ExtractAllText() to get contents
5. Compare the extracted text or implement custom logic
6. Save results or generate reports as needed
7. Close document objects to free resources

Setting Your License

To use IronPDF without watermarks, set your license key:

IronPdf.License.LicenseKey = "YOUR-LICENSE-KEY";

IronPdf.License.LicenseKey = "YOUR-LICENSE-KEY";

Visit the licensing page for details on available options. Download IronPDF's free trial and set up PDF files using professional-grade comparison capabilities.

Best Practices

1. Accept all revisions before comparison if working with tracked changes
2. Develop error handling for crucial file operations
3. Enhance performance by creating page-specific comparisons for large documents
4. Run comparisons asynchronously for multiple PDF documents
5. Update your library regularly for the latest features and support

Conclusion

IronPDF simplifies the process to compare two PDF files using C# with its intuitive API and powerful extraction capabilities. From basic comparison to creating detailed reports, IronPDF provides all the tools needed for robust document comparison workflows.

The combination of easy installation, comprehensive functionality, and cross-platform support makes IronPDF the ideal choice for developers implementing PDF comparison in .NET applications. Whether you're developing document management systems or version control solutions, IronPDF's straightforward approach gets you running quickly.

Start your free trial of IronPDF today and implement professional PDF comparison features. With full technical support and extensive documentation, you'll have production-ready functionality in hours. Refer to the complete documentation to learn more about advanced features.