sensenet has a powerful preview feature available on the admin ui (and through API as well). It enables to preview multiple types of documents right in the browser (no download or any third party application needed).
Preview in action
Once a file is uploaded, the system (depending on the configuration) starts generating preview images in the background. After the process is finished, the preview of the file is accessible by double clicking the content or by selecting "preview" from the action menu.
The preview tool has some basic features like zoom or document rotation accessible by the action buttons in the top toolbar. Here you can also toogle the thumbnail pane or add comments to any part of the document.
To add a comment to a document, open preview and click the "toogle commments" button at the top right corner. Now place the marker in the document by single clicking the desired place. Write your comment in the text field and click submit. That's it. Existing comments (from all users) will appear in the comment list along with markers in the document (only in preview mode).