Unfortunately, the API doesn’t return converted html content as stream. However, I have logged a request in our Issue Tracking system as VIEWERNET-860 for implementation of this feature. We shall further investigate it and in case of any information, we will notify you here.
Can you please tell us why and how you want to get html content as a stream? You can provide us your particular scenario in which you require html content to be returned as stream. This will help us to better understand the problem and offer a suitable solution.
Secondly, please note that GroupDocs.Viewer for Java is ported from GroupDocs.Viewer for .NET therefore html content as stream is also not supported in Java API.