Why does my header overlap with my content?

Please take a look at the TableHeader example adapted for iText 7.

In this example, I create a document with some "Hello World" content:

public void createPdf(String dest) throws IOException {
    PdfDocument pdfDoc = new PdfDocument(new PdfWriter(dest));
    Document doc = new Document(pdfDoc, PageSize.A4);
    TableHeaderEventHandler handler = new TableHeaderEventHandler(doc);
    pdfDoc.addEventHandler(PdfDocumentEvent.END_PAGE, handler);
    doc.setMargins(20 + handler.getTableHeight(), 36, 36, 36);
    for (int i = 0; i 

As you can see, I also define a TableHeaderEventHandler. I use this event as a page event:

    pdfDoc.addEventHandler(PdfDocumentEvent.END_PAGE, handler);

What does this mean? Let's take a look at the implementation of this event handler:

   public class TableHeaderEventHandler implements IEventHandler {
        protected Table table;
        protected float tableHeight;
        protected Document doc;

        public TableHeaderEventHandler(Document doc) {
            this.doc = doc;
            table = new Table(1);
            table.setWidth(doc.getPdfDocument().getDefaultPageSize().getRight() - doc.getPdfDocument().getDefaultPageSize().getLeft() - doc.getLeftMargin() - doc.getRightMargin());
            table.addCell("Header row 1");
            table.addCell("Header row 2");
            table.addCell("Header row 3");
            TableRenderer renderer = (TableRenderer) table.createRendererSubTree();
            renderer.setParent(new Document(new PdfDocument(new PdfWriter(new ByteArrayOutputStream()))).getRenderer());
            tableHeight = renderer.layout(new LayoutContext(new LayoutArea(0, PageSize.A4))).getOccupiedArea().getBBox().getHeight();
        }

        @Override
        public void handleEvent(Event event) {
            PdfDocumentEvent docEvent = (PdfDocumentEvent) event;
            PdfDocument pdfDoc = docEvent.getDocument();
            PdfPage page = docEvent.getPage();
            PdfCanvas canvas = new PdfCanvas(page.newContentStreamBefore(), page.getResources(), pdfDoc);
            Rectangle rect = new Rectangle(pdfDoc.getDefaultPageSize().getX() + doc.getLeftMargin(),
                    pdfDoc.getDefaultPageSize().getTop() - doc.getTopMargin(), 100, getTableHeight());
            new Canvas(canvas, pdfDoc, rect)
                    .add(table);
        }

        public float getTableHeight() {
            return tableHeight;
        }
    }

You need the height of the table to set the right top margin for the document. When you create a TableHeaderEventHandler, the height of the rendered table is calculated in the constructor using layout() method. According to this value you will use this line:

doc.setMargins(20 + handler.getTableHeight(), 36, 36, 36);

The result looks like this:

Header table and content: no overlap. Page 1.

Header table and content: no overlap. Page 2.

Now the content does not overlap the header.

Click Why does my header overlap with my content? - iText 5 PDF Library Explained if you want to see how to answer this question in iText 5.