Merge Multiple PDFs in C# with Spire.PDF

In practical project development, we often encounter requirements to merge multiple PDF files into a single document or extract specific pages from multiple PDFs to form a new PDF. This article will show you how to use the Spire.PDF for .NET library to implement two functions:

• Merge multiple complete PDF documents
• Merge specified pages from multiple PDFs

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1. Install Spire.PDF

Before getting started, you need to install Spire.PDF for .NET. You can install it via NuGet:

Install-Package Spire.PDF

Or search for Spire.PDF in Visual Studio's "Manage NuGet Packages" and install it.

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2. Merge Multiple PDF Documents (Basic Scenario)

This is suitable for concatenating multiple PDF files in sequence into one document.

using Spire.Pdf;

namespace MergePDFs
{
    class Program
    {
        static void Main(string[] args)
        {
            // PDF documents to be merged
            string[] files = new string[] {"Example1.pdf", "Example2.pdf", "Example3.pdf"};

            // Merge PDF documents
            PdfDocumentBase pdf = PdfDocument.MergeFiles(files);

            // Save
            pdf.Save("MergedPDF.pdf", FileFormat.PDF);
        }
    }
}

✅ Advantages: Simple code, stable execution, suitable for batch processing.
⚠️ Note: Ensure file paths exist and are readable.

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3. Merge Specified Pages (Advanced Scenario)

In practical work, you often need to merge specific pages from different PDFs (e.g., page 2 of a contract + page 3 of an approval form).

using Spire.Pdf;

namespace MergePDFs
{
    class Program
    {
        static void Main(string[] args)
        {
            // PDF documents to be merged
            string[] files = new string[] {"Example1.pdf", "Example2.pdf"};

            // Load each PDF document
            PdfDocument[] pdfs = new PdfDocument[files.Length];
            for (int i = 0; i < files.Length; i++)
            {
                pdfs[i] = new PdfDocument(files[i]);
            }

            // Create a new PdfDocument object
            PdfDocument newPDF = new PdfDocument();

            // Merge pages 2 and 3 from the first document and page 1 from the second document
            newPDF.InsertPageRange(pdfs[0], 1, 2);
            newPDF.InsertPage(pdfs[1], 0);

            // Save the new PDF file
            newPDF.SaveToFile("ExtractedPDFPages.pdf");
        }
    }
}

📌 Applicable scenarios: Cross-document content integration, report page reorganization.

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4. Notes

• Page Indexing: Spire.PDF uses 0-based page indexing, unlike some software that starts from 1.
• Large File Handling: When merging many or large PDFs, use using statements to ensure proper resource release.
• Supported Formats: Spire.PDF supports PDF 1.0 ~ PDF 1.7 formats, as well as some PDF/A formats.

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With the above code, you can implement basic PDF merging in C# and also flexibly control page order and skip specific pages to meet complex business scenarios.