Thanks for taking interest in GroupDocs.Viewer for .NET API.
Yes, you can use GroupDocs.Viewer for .NET API to view PDF and Office documents in your SharePoint application. In fact, GroupDocs.Viewer for .NET is a platform independent back-end API that renders the source document into HTML pages or images. You can load these generated HTML pages or images in your application to preview the content of the documents.
Does the SharePoint Plugin for GroupDocs.Viewer work to preview documents in sharepoint lists?
Our sample SharePoint Web Part document viewer application displays document which are located in a storage directory placed on the disk. However, as the application is open source, you can modify it and load the documents from SharePoint List.
Also I need to be able to add annotations to the same SharePoint list documents
In case any thing is not clear at your end, please do let us know.