How to add text as a header or footer?

class MyFooter extends PdfPageEventHelper {
    public void onEndPage(PdfWriter writer, Document document) {
        PdfContentByte cb = writer.getDirectContent();
        ColumnText.showTextAligned(cb, Element.ALIGN_CENTER, footer(),
            (document.right() - document.left()) / 2 + document.leftMargin(),
            document.top() + 10, 0);
    }
    private Phrase footer() {
        Font ffont = new Font(Font.FontFamily.UNDEFINED, 5, Font.ITALIC);
        Phrase p = new Phrase("this is a footer");
        return p;
    }
}
...
Document document = new Document(PageSize.A4);
PdfWriter writer = PdfWriter.getInstance(document, new FileOutputStream(file));
writer.setPageEvent(new MyFooter());

The problem you report can not be reproduced. I have taken your example and I create the TextFooter example for iText 7. You should implement IEventHandler like this:

protected class TextFooterEventHandler implements IEventHandler {
    protected Document doc;
    public TextFooterEventHandler(Document doc) {
        this.doc = doc;
    }
    @Override
    public void handleEvent(Event event) {
        PdfDocumentEvent docEvent = (PdfDocumentEvent) event;
        PdfCanvas canvas = new PdfCanvas(docEvent.getPage());
        Rectangle pageSize = docEvent.getPage().getPageSize();
        canvas.beginText();
        try {
            canvas.setFontAndSize(PdfFontFactory.createFont(FontConstants.HELVETICA_OBLIQUE), 5);
        } catch (IOException e) {
            e.printStackTrace();
        }
        canvas.moveText((pageSize.getRight() - doc.getRightMargin() - (pageSize.getLeft() + doc.getLeftMargin())) / 2 + doc.getLeftMargin(), pageSize.getTop() - doc.getTopMargin() + 10)
                .showText("this is a header")
                .moveText(0, (pageSize.getBottom() + doc.getBottomMargin()) - (pageSize.getTop() + doc.getTopMargin()) - 20)
                .showText("this is a footer")
                .endText()
                .release();
    }
}

There is no ColumnText class in iText 7 anymore, so we use PdfCanvas instead and add content to it. To get the desired positions on the top and bottom of the page we use a PageSize instance and the appropriate methods.

There was also an error in your footer() method, you didn’t apply a font to the Phrase. However in iText 7 such method will look like this (Phrases don’t exist anymore and fonts are defined in a different way):

private Paragraph footer() throws IOException {
    PdfFont ffont = PdfFontFactory.createFont(FontConstants.HELVETICA);
    Paragraph p = new Paragraph("this is a footer")
            .setFont(ffont)
            .setFontSize(5)
            .setItalic();
    return p;
}

Now when we look at the resulting PDF, we clearly see the text that was added as a header and a footer to each page.

Click How to add text as a header or footer? if you want to see how to answer this question in iText 5.