public void MergeDocument()
{
    //Initialize the SDK library
    //You have to call this function before you can call any PDF processing functions.
    PdfCommon.Initialize();

    //Open and load a PDF document in which will be merged other files
    using (var mainDoc = PdfDocument.Load(@"c:\test001.pdf"))
    {
        //Open one PDF document.
        using (var doc = PdfDocument.Load(@"c:\doc1.pdf"))
        {
            //Import all pages from document
            mainDoc.Pages.ImportPages(
                doc,
                string.Format("1-{0}", doc.Pages.Count),
                mainDoc.Pages.Count
                );
        }

        //Open another PDF document.
        using (var doc = PdfDocument.Load(@"c:\doc2.pdf"))
        {
            //Import all pages from document
            mainDoc.Pages.ImportPages(
                doc,
                string.Format("1-{0}", doc.Pages.Count),
                mainDoc.Pages.Count
                );
        }

        mainDoc.Save(@"c:\ResultDocument.pdf", SaveFlags.NoIncremental);


    }
    //Release all resources allocated by the SDK library
    PdfCommon.Release();
}