如何使用objective C查看PDF文件中的数字签名

I have a PDF viewer in my app. I am simply loading the PDF file into a UIWebView. But when the PDF contains digital signature, it is not displayed in WebView. All other content of the PDF is displayed. Only signature area is left blank. I checked with the Preview, and Safari in OSX also, and the result is the same. I have a PDF viewer in my app. I am simply load