Thank you for your cooperation.
GroupDocs.Annotation for .NET API MVC & WebForms Frond-End demo applications are open source and can be updated according to your requirements, currently both MVC & WebForms front-end showcase applications are developed to to view the output as images only instead of PDF file output which should required to customize while viewing in the front-end accordingly, however there are also multiple options in GroupDocs.Annotation API to export the output document
in different formats supported.
Please click here
for the exact source code lines responsible for rendering the output as images in GroupDocs.Annotation front-end viewer.
If you will face any issue or you will have any other query, please feel free to ask.