How to Add Content to an Existing PDF File Using iText

How can I modify an existing PDF by adding new content using the iText library?

Document document = reader.read(input);
document.add(new Paragraph("my timestamp"));
writer.write(document, output);

iText is a powerful library that allows you to create and manipulate PDF documents in Java. To add content to an existing PDF, you need to follow a few steps, which involve reading the PDF, adding the content, and then writing it out.

import com.itextpdf.text.*;
import com.itextpdf.text.pdf.*;

public void addContentToExistingPDF(String src, String dest) throws Exception {
    PdfReader reader = new PdfReader(src);
    PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
    PdfContentByte canvas = stamper.getOverContent(1);
    BaseFont bf = BaseFont.createFont();
    canvas.beginText();
    canvas.setFontAndSize(bf, 12);
    canvas.setTextMatrix(100, 700); // Set position
    canvas.showText("Timestamp: " + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date()));
    canvas.endText();
    stamper.close();
    reader.close();
}

Causes

• Misunderstanding the iText API's document manipulation process.
• Confusion about the roles of PdfReader and PdfWriter in the context of PDF modification.

Solutions

• Use `PdfReader` for reading existing PDF files.
• Utilize `PdfStamper` to modify existing content and add new elements.
• For writing out the modified document, ensure you use `PdfWriter` correctly.

Mistake: Not using PdfStamper for modifications.

Solution: Always use PdfStamper when you need to make changes to an existing PDF.

Mistake: Failing to close PdfReader and PdfStamper properly, leading to file locks or corrupt PDFs.

Solution: Ensure you always close both PdfReader and PdfStamper in a finally block or use try-with-resources.