//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 with forms
using (var mainDoc = PdfDocument.Load(@"d:\00\otpt4.pdf"))
{
	//Open another PDF document.
	using (var doc = PdfDocument.Load(@"d:\00\facture.pdf"))
	{
		//Import all pages from second document into first one
		mainDoc.Pages.ImportPages(
			doc,
			string.Format("1-{0}", doc.Pages.Count),
			mainDoc.Pages.Count
			);
	}

	//Save a copy of a merged document
	mainDoc.Save(@"d:\00\result.pdf", SaveFlags.NoIncremental);

}

PdfCommon.Initialize();

//Create a new document
using (var newDoc = PdfDocument.CreateNew())
{
	//Open PDF document with forms
	using (var doc1 = PdfDocument.Load(@"d:\00\otpt4.pdf"))
	{
		//Open another PDF document.
		using (var doc2 = PdfDocument.Load(@"d:\00\facture.pdf"))
		{
			//Import all pages from first document into newly created document
			newDoc.Pages.ImportPages(
				doc1,
				string.Format("1-{0}", doc1.Pages.Count),
				newDoc.Pages.Count
				);

			//Import all pages from second document into newly created document
			newDoc.Pages.ImportPages(
				doc2,
				string.Format("1-{0}", doc2.Pages.Count),
				newDoc.Pages.Count
				);
		}

		//Save a copy of a document
		newDoc.Save(@"d:\00\result.pdf", SaveFlags.NoIncremental);
	}
}