Comparing Two PDF Documents in C#

In daily software development and office workflows, PDF document comparison stands as a high-frequency requirement—common use cases include contract version iteration verification, technical document peer reviews, and financial report consistency validation. While visual comparison tools like Adobe Acrobat are widely available on the market, implementing PDF comparison programmatically offers unparalleled advantages in supporting scalable automation and tailored business workflows. This article demonstrates how to compare two PDF documents efficiently using Free Spire.PDF for .NET, a free and lightweight .NET library.

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I. Core Component: The PdfComparer Class

PdfComparer is a high-level encapsulated class provided natively by Free Spire.PDF for .NET, engineered specifically for PDF content comparison scenarios. It eliminates the need for manual text extraction by automatically analyzing core content differences between two PDF files, including text additions, deletions, and modifications. The class directly outputs comparison results as a new, annotated PDF document, streamlining the post-comparison review process.

Visual Presentation of Comparison Results

The generated comparison report adopts intuitive visual cues to distinguish content changes, ensuring clear and quick identification of differences:

• Fully consistent PDFs: No highlight markers are added; the document displays the original content in its entirety.
• Newly added content: Highlighted in yellow for instant visibility.
• Removed content: Marked with red highlights for clear differentiation.

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II. Install Free Spire.PDF for .NET

The most straightforward method is installation via the NuGet Package Manager, following these step-by-step instructions:

1. Open your project in Visual Studio; right-click the project in the Solution Explorer and select Manage NuGet Packages.
2. Navigate to the Browse tab, search for FreeSpire.PDF, and install the latest stable version.

For command-line enthusiasts or CI/CD pipeline integration, use the following NuGet CLI command:

Install-Package FreeSpire.PDF

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III. Step-by-Step Guide to Comparing PDFs in C

Core Steps

Free Spire.PDF simplifies PDF comparison to a four-step process with zero manual intervention in text parsing or page matching. The streamlined workflow reduces development complexity and minimizes potential errors:

1. Instantiate PdfDocument objects and load the two target PDF files respectively.
2. Initialize a PdfComparer instance, passing the two loaded PdfDocument objects as parameters.
3. Invoke the PdfComparer.Compare() method and specify the file path to save the comparison result PDF.
4. Explicitly close PdfDocument instances to release occupied memory and prevent memory leaks.

Important Note: The free version has a page limit for document processing. However, this limitation is negligible for most basic comparison tasks in development, testing, or small-scale office scenarios.

Complete C# Code Example

The following concise code snippet implements end-to-end PDF content comparison with minimal lines of code:

using Spire.Pdf;
using Spire.Pdf.Comparison;

namespace PDFComparisonDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            // Load the two PDF documents to be compared
            PdfDocument sourcePdf = new PdfDocument();
            sourcePdf.LoadFromFile("Sample1.pdf");

            PdfDocument targetPdf = new PdfDocument();
            targetPdf.LoadFromFile("Sample2.pdf");

            // Initialize PdfComparer with the two documents
            PdfComparer pdfComparer = new PdfComparer(sourcePdf, targetPdf);

            // Execute comparison and save the annotated result
            pdfComparer.Compare("PDFComparison_Result.pdf");

            // Release resources to avoid memory leaks
            sourcePdf.Close();
            targetPdf.Close();
        }
    }
}

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Conclusion

The PDF comparison solution based on Free Spire.PDF for .NET’s PdfComparer class offers a streamlined, efficient approach for C# developers. By abstracting complex underlying comparison logic, it enables developers to implement PDF comparison features with minimal code. Unlike plain text difference logs, the annotated PDF output provides a visual, human-readable way to review changes, making it ideal for integrating into document management systems, version control workflows, and automated audit processes.