I have a digital sign pdf with 2 form field having the same name. when i try fill up the field using PDFBOX and open it in Acrobat the field content is empty. It only will show the content when i click on the field.

This my code

    File inFile= new File("pdfFile/Untitled_sign.pdf");
    OutputStream outFile = new FileOutputStream("pdfFile/Untitled_sign_2.pdf");
    PDDocument pdfDocument = PDDocument.load(inFile);

    PDAcroForm acroForm = pdfDocument.getDocumentCatalog().getAcroForm();

    Set<COSDictionary> cosDis=  new HashSet<>();
    if (acroForm != null) {
        List<PDField> fields = acroForm.getFields();
        //acroForm.setNeedAppearances(true);
        for (PDField field : fields) {
            if(field instanceof PDTextField) {
                System.out.println(field.getFullyQualifiedName());
                if(field.getFullyQualifiedName().equals("Text1")) {
                    try {
                        field.setValue("100");
                        System.out.println("New value: "   field.getValueAsString());
                        
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    field.getCOSObject().setNeedToBeUpdated(true);
                    cosDis.add(field.getCOSObject());
                }
            }
        }
        acroForm.getCOSObject().setNeedToBeUpdated(true);
        cosDis.add(acroForm.getCOSObject());
    }
      pdfDocument.saveIncremental(outFile,cosDis);

here the pdf

here my final code.

    PDDocument pdfDocument = PDDocument.load(inFile);
    PDAcroForm acroForm = pdfDocument.getDocumentCatalog().getAcroForm();
    Set<COSDictionary> cosDis=  new HashSet<>();
    if (acroForm != null) {
        List<PDField> fields = acroForm.getFields();
        for (PDField field : fields) {
            if(field instanceof PDTextField) {
                if(field.getFullyQualifiedName().equals("Text1")) {
                    try {
                        field.setValue("200");
                        System.out.println("New value: "   field.getValueAsString());
                        
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    field.getCOSObject().setNeedToBeUpdated(true);
                    cosDis.add(field.getCOSObject());
                    for(PDAnnotationWidget wed: field.getWidgets()) {
                        wed.getAppearance().getCOSObject().setNeedToBeUpdated(true);
                        wed.getCOSObject().setNeedToBeUpdated(true);
                         cosDis.add(wed.getAppearance().getCOSObject());
                         cosDis.add(wed.getCOSObject());
                    }
                    COSDictionary fieldDictionary = field.getCOSObject();
                    COSDictionary dictionary = (COSDictionary) fieldDictionary.getDictionaryObject(COSName.AP);
                    if(dictionary!=null) {
                       dictionary.setNeedToBeUpdated(true);
                       cosDis.add(dictionary);
                       COSStream stream = (COSStream) dictionary.getDictionaryObject(COSName.N);
                       stream.setNeedToBeUpdated(true);
                       cosDis.add(stream);
                    }else {
                        COSArray dictionary2 = (COSArray) fieldDictionary.getDictionaryObject(COSName.KIDS);
                        for (int i = 0; i < dictionary2.size(); i  )
                        {
                            COSDictionary kidAp=(COSDictionary) ((COSDictionary)dictionary2.getObject(i)).getDictionaryObject(COSName.AP);
                            if(kidAp!=null) {
                                kidAp.setNeedToBeUpdated(true);
                                cosDis.add(kidAp);
                                COSStream kidApstream = (COSStream) kidAp.getDictionaryObject(COSName.N);
                                kidApstream.setNeedToBeUpdated(true);
                                cosDis.add(kidApstream);
                            }
                        }
                    }
                    while (fieldDictionary != null)
                    {
                        fieldDictionary.setNeedToBeUpdated(true);
                        cosDis.add(fieldDictionary);
                        fieldDictionary = (COSDictionary) fieldDictionary.getDictionaryObject(COSName.PARENT);
                    }
                }
            }
        }
        acroForm.setSignaturesExist(true);
        acroForm.setAppendOnly(true);
        acroForm.getCOSObject().setDirect(true);
        acroForm.getCOSObject().setNeedToBeUpdated(true);
        
        //acroForm.setNeedAppearances(true);
        COSObject pdfFields = acroForm.getCOSObject().getCOSObject(COSName.FIELDS);
        if (pdfFields != null) {
            pdfFields.setNeedToBeUpdated(true);
        }
        pdfDocument.getDocumentCatalog().getCOSObject().setNeedToBeUpdated(true);
        cosDis.add(pdfDocument.getDocumentCatalog().getCOSObject());
        cosDis.add(acroForm.getCOSObject());
    }
      pdfDocument.saveIncremental(outFile,cosDis);

Hope this help. ;)