I found the answer:
Instead of the 2nd Method, add more files to the first array of input files.
public static void CombineMultiplePDFs(string[] fileNames, string outFile)
{
// step 1: creation of a document-object
Document document = new Document();
//create newFileStream object which will be disposed at the end
using (FileStream newFileStream = new FileStream(outFile, FileMode.Create))
{
// step 2: we create a writer that listens to the document
PdfCopy writer = new PdfCopy(document, newFileStream );
if (writer == null)
{
return;
}
// step 3: we open the document
document.Open();
foreach (string fileName in fileNames)
{
// we create a reader for a certain document
PdfReader reader = new PdfReader(fileName);
reader.ConsolidateNamedDestinations();
// step 4: we add content
for (int i = 1; i <= reader.NumberOfPages; i++)
{
PdfImportedPage page = writer.GetImportedPage(reader, i);
writer.AddPage(page);
}
PRAcroForm form = reader.AcroForm;
if (form != null)
{
writer.CopyAcroForm(reader);
}
reader.Close();
}
// step 5: we close the document and writer
writer.Close();
document.Close();
}//disposes the newFileStream object
}