How to Merge Multiple (Two+) PDF Documents Together in C# .NET Framework

Writing code to merge PDF documents at scale is no simple task. Thankfully, we don’t need to worry about that when we can lean on a free low-code API solution to get the job done.

Using the below code, we can easily take advantage of a free API that’ll merge 10+ PDF documents together for us at once, preserving the order that we upload our files in.

We can authorize our API calls with a free-tier API key, and this will allow us to make up to 800 API calls per month with no additional commitments (perfect for smaller-scale projects and/or experimenting with new solutions).

Let’s kick things off by installing the SDK. We can install via NuGet by running this command in our Package Manager console:

Install-Package Cloudmersive.APIClient.NET.DocumentAndDataConvert -Version 3.4.2

Right after that, we can copy the below examples into our file to call the function:

using System;
using System.Diagnostics;
using Cloudmersive.APIClient.NET.DocumentAndDataConvert.Api;
using Cloudmersive.APIClient.NET.DocumentAndDataConvert.Client;
using Cloudmersive.APIClient.NET.DocumentAndDataConvert.Model;

namespace Example
{
    public class MergeDocumentPdfMultiExample
    {
        public void main()
        {
            // Configure API key authorization: Apikey
            Configuration.Default.AddApiKey("Apikey", "YOUR_API_KEY");

            var apiInstance = new MergeDocumentApi();
            var inputFile1 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | First input file to perform the operation on.
            var inputFile2 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Second input file to perform the operation on.
            var inputFile3 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Third input file to perform the operation on. (optional) 
            var inputFile4 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Fourth input file to perform the operation on. (optional) 
            var inputFile5 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Fifth input file to perform the operation on. (optional) 
            var inputFile6 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Sixth input file to perform the operation on. (optional) 
            var inputFile7 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Seventh input file to perform the operation on. (optional) 
            var inputFile8 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Eighth input file to perform the operation on. (optional) 
            var inputFile9 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Ninth input file to perform the operation on. (optional) 
            var inputFile10 = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Tenth input file to perform the operation on. (optional) 

            try
            {
                // Merge Multple PDF Files Together
                byte[] result = apiInstance.MergeDocumentPdfMulti(inputFile1, inputFile2, inputFile3, inputFile4, inputFile5, inputFile6, inputFile7, inputFile8, inputFile9, inputFile10);
                Debug.WriteLine(result);
            }
            catch (Exception e)
            {
                Debug.Print("Exception when calling MergeDocumentApi.MergeDocumentPdfMulti: " + e.Message );
            }
        }
    }
}

Now we can load in our PDF files and begin merging. It’s just that simple — no more code required!