Thank you for giving a try to GroupDocs.Annotation for .NET API.
We are sorry to hear that you are facing such issue. Yes you can load PDF document using GroupDocs.Annotation for .NET sample front-end project for this you need a license file. please click here to get free temporary license.
Please find below some useful resources to quickly get started with GroupDocs.Annotation API.
- GroupDocs.Annotation Docs: http://www.groupdocs.com/docs/display/annotationnet/Home
- GroupDocs.Annotation Videos: https://www.youtube.com/channel/UC5zdCdyWw7gP2Y0_H6klwOA
- GroupDocs.Annotation Examples: https://github.com/groupdocs-annotation/GroupDocs.Annotation-for-.NET
If you will need any help or you will have any other questions please feel free to ask.