1. 在项目中添加iTextPDF库的依赖。在这里我使用了iText7库。

implementation 'com.itextpdf:itext7-core:7.1.15'
implementation 'com.itextpdf:itext7-sign:7.1.15'

2. 在要添加签名的PDF文档中添加FormField。FormField可以用来添加签名数字，并且iText7库也提供了几种类型的签名域来覆盖使用场景。

PdfAcroForm acroForm = PdfAcroForm.getAcroForm(pdfDocument, true);
PdfSignatureFormField field = PdfFormField.createSignature(pdfDocument, rect);
field.setFieldName("signature");
field.getWidgets().get(0).setHighlightMode(PdfAnnotation.HIGHLIGHT_INVERT);
acroForm.addField(field);

3. 为签名设置属性和位置。

field.setValue("digitally signed by Jerome");
field.setFieldName("Signature");
field.setName("Jerome");
field.setLocation("Ghent");
field.setReason("Testing iText signing");
field.setContactInfo("test@example.com");
PdfDictionary dict = new PdfDictionary();
dict.put(PdfName.Filter, PdfName.Adobe_PPKLite);
dict.put(PdfName.SubFilter, PdfName.Adbe_pkcs7_detached);
field.setCryptoDictionary(dict);

4. 创建签名的证书和密钥对，然后使用私钥对PDF进行签名。

String keystorePath = "/path/to/keystore.jks";
String password = "your_password";
String alias = "your_alias";
KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());
ks.load(new FileInputStream(keystorePath), password.toCharArray());

PrivateKey pk = (PrivateKey) ks.getKey(alias, password.toCharArray());
Certificate[] chain = ks.getCertificateChain(alias);

PdfSigner signer = new PdfSigner(new PdfReader("input.pdf"), os, new StampingProperties());
PdfSignatureAppearance appearance = signer.getSignatureAppearance();
appearance.setReason("Test");
appearance.setLocation("Ghent");
appearance.setPageNumber(1);
appearance.setCertificate(chain[0]);

SignUtils signUtils = new SignUtils(pk, chain, "SHA-256