at least one signature is invalid in pdf signing

when I am signing a PDF using IText with e-token then signed pdf is showing'at least one signature is invalid' in Acrobat Adobe Reader.I am using valid e-token. Below is the code for signing Pdf.

//path of destination file    
String destFile = "D://sign_test.pdf";  
// load key store    
KeyStore ks = KeyStore.getInstance("Windows-MY");       
ks.load(null, "password".toCharArray());    
Enumeration enumeration = ks.aliases();    
String alias = null;    
while(enumeration.hasMoreElements()){  
alias = (String)enumeration.nextElement();  
if(alias.equalsIgnoreCase("alias of the certificate"))// to get specific certificate from keystore  
break;  
}  
PrivateKey pk = (PrivateKey)ks.getKey(alias, "password".toCharArray());  Certificate[] chain = ks.getCertificateChain(alias);  
PdfReader reader = new PdfReader(sourceFile);  
FileOutputStream os = new FileOutputStream(destFile);  
PdfStamper stamper = PdfStamper.createSignature(reader, os, '\0');  
PdfSignatureAppearance appearance = stamper .getSignatureAppearance();  
appearance.setReason("I've written this.");  
appearance.setLocation("Foobar");  
appearance.setVisibleSignature(new Rectangle(72, 732, 144,      780),1,"first");  
ExternalSignature es = new PrivateKeySignature(pk, "SHA-256", "SunMSCAPI");    
ExternalDigest digest = new ProviderDigest("SunMSCAPI");  
MakeSignature.signDetached(appearance, digest, es, chain, null, null,    null, 0, CryptoStandard.CMS);    
stamper.close();