Flatten.java

import java.io.File;
import java.io.IOException;
import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.pdmodel.interactive.form.PDAcroForm;
import org.apache.pdfbox.pdmodel.interactive.form.PDField;

public class Flatten
{
    public static void main(String[] args) throws IOException {
        PDDocument doc = PDDocument.load(new File("interactiveform.pdf"));
        PDAcroForm acroForm = doc.getDocumentCatalog().getAcroForm();
        for (PDField field : acroForm.getFields())
            {
                field.setReadOnly(true);
            }
        doc.getDocumentCatalog().getAcroForm().flatten();
        doc.save("new.pdf");
        doc.close();
    }
}