我的服务器拒绝了请求，因为飞碟库中缺少一个用户代理。
为了解决这个问题，我重载了itextureagent，手动添加了一个用户代理：

public class ResourceLoaderUserAgent extends ITextUserAgent
{
    ResourceLoaderUserAgent(ITextOutputDevice outputDevice)
    {
        super(outputDevice);
    }

    @Override
    protected InputStream openStream(String uri) throws IOException
    {
        URLConnection connection = new URL(uri).openConnection();
        connection.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.11 (KHTML, like Gecko) Chrome/23.0.1271.95 Safari/537.11");
        connection.connect();
        return connection.getInputStream();
    }
}

我是这样用的：

ITextRenderer iTextRenderer = new ITextRenderer();

    ResourceLoaderUserAgent callback = new ResourceLoaderUserAgent(iTextRenderer.getOutputDevice());
    callback.setSharedContext(iTextRenderer.getSharedContext());
    iTextRenderer.getSharedContext().setUserAgentCallback(callback);