Why does the function to concatenate / merge PDFs cause issues in some cases?

public static void concatenatePdfs(ListFile > listOfPdfFiles, File outputFile) throws DocumentException, IOException {
  Document document = newDocument();
  FileOutputStream outputStream = newFileOutputStream(outputFile);
  PdfWriter writer = PdfWriter.getInstance(document, outputStream);
  document.open();
  PdfContentByte cb = writer.getDirectContent();
  for (File inFile: listOfPdfFiles) {
    PdfReader reader = new PdfReader(inFile.getAbsolutePath());
    for (int i = 1; i reader.getNumberOfPages(); i++) {
      document.newPage();
      PdfImportedPage page = writer.getImportedPage(reader, i);
      cb.addTemplate(page, 0, 0);
    }
  }
  document.close();
}

There are errors once in a while because you are using the wrong method to concatenate documents. You should not use PdfWriter to concatenate (or merge) PDF documents. That is wrong because:

• 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 was done using PdfCopy in iText 5. In iText 7 we create a new PdfDocument and use the copyPagesTo() method, like this:

PdfWriter writer = new PdfWriter(toFile);
writer.setSmartMode(isSmartModeOn);
PdfDocument pdfDoc = new PdfDocument(writer);
pdfDoc.initializeOutlines();
PdfDocument addedDoc = new PdfDocument(new PdfReader(fromFile));
addedDoc.copyPagesTo(1, addedDoc.getNumberOfPages(), pdfDoc);
addedDoc.close();
pdfDoc.close();

Click this link if you want to see how to answer this question in iText 5.