function that can use iText to concatenate / merge pdfs together - causing some issues

Solution 1:

There are errors once in a while because you are using the wrong method to concatenate documents. Please read chapter 6 of my book and you'll notice that using PdfWriter to concatenate (or merge) PDF documents is wrong:

• You completely ignore the page size of the pages in the original document (you assume they are all of size A4),
• You ignore page boundaries such as the crop box (if present),
• You ignore the rotation value stored in the page dictionary,
• You throw away all interactivity that is present in the original document, and so on.

Concatenating PDFs is done using PdfCopy, see for instance the FillFlattenMerge2 example:

Document document = new Document();
PdfCopy copy = new PdfSmartCopy(document, new FileOutputStream(dest));
document.open();
PdfReader reader;
String line = br.readLine();
// loop over readers
    // add the PDF to PdfCopy
    reader = new PdfReader(baos.toByteArray());
    copy.addDocument(reader);
    reader.close();
// end loop
document.close();

There are other examples in the book.

Solution 2:

In case anyone is looking for it, using Bruno Lowagie's correct answer above, here is the version of the function that does not seem to have the page flipping issue i described above:

public static void concatenatePdfs(List<File> listOfPdfFiles, File outputFile) throws DocumentException, IOException {
        Document document = new Document();
        FileOutputStream outputStream = new FileOutputStream(outputFile);
        PdfCopy copy = new PdfSmartCopy(document, outputStream);
        document.open();
        for (File inFile : listOfPdfFiles) {
            PdfReader reader = new PdfReader(inFile.getAbsolutePath());
            copy.addDocument(reader);
            reader.close();
        }
        document.close();
}