Thank you for your interest in GroupDocs.
1. GroupDocs.Annotation contains GroupDocs.Viewer inside and uses it when displaying and annotating the documents. You cannot use your own document viewer along with GroupDocs.Annotation while annotating the documents, but of course you can use both of them for different purposes: your document viewer for displaying-only and GroupDocs.Annotation for annotating.
2. We have 3 implementations of GroupDocs.Annotation: for .NET and for Java (these are standalone libraries) and as a Cloud service (Apps and API). If you want a solution which allows to store and processes all the documents at yours own server, you need to choose between GroupDocs.Annotation for .NET and GroupDocs.Annotation for Java.
3. Yes, we have the tutorials, here are links: GroupDocs.Annotation for .NET Library, GroupDocs.Annotation for Java Library.
4. All our libraries work in trial-mode if you don’t specify a license file. This trial has unlimited time period, but limited features. However, if you have strong interest in GroupDocs.Annotation and want to try it to the fullest, you can request 30-day evaluation license on the Sales forum.
If you will have more questions please feel free to contact us.