static void SimpleImport()
{
    using (var dst = PdfDocument.Load(@"d:\0\1.pdf"))
    {
        var src = PdfDocument.Load(@"d:\0\100ThousandPages.pdf");
        int cnt = src.Pages.Count;
        for (int i = 0, j=0; i < cnt; i++, j++)
        {
            //Process sourcePage
            var scrPage = src.Pages[i].Handle;
            //Import page
            dst.Pages.ImportPages(src, (i+1).ToString(), dst.Pages.Count - 1);
            //DisposePage
            src.Pages[i].Dispose();

            if(j>20)
            {
                j = 0;
                src.Dispose();
                src = PdfDocument.Load(@"d:\0\100ThousandPages.pdf");
                Console.WriteLine(i);
            }
        }
        src.Dispose();
        dst.Save(@"d:\0\imported.pdf", Patagames.Pdf.Enums.SaveFlags.NoIncremental);
    }
}