GemBox.Pdf
.NET PDF Library

static void Main()
{
    // Set license key to use GemBox.Pdf in Free mode.
    ComponentInfo.SetLicense("FREE-LIMITED-KEY");

    // Load PDF documents.
    using var document = PdfDocument.Load("Document1.pdf");
    using var document2 = PdfDocument.Load("Document2.pdf");

    // Merge document2 into document.
    document.Pages.AddClone(document2.Pages);

    // Save the merged PDF document.
    document.Save("Merged Documents.pdf");
}