For now, the static methods of ExtensibilityManager class are the only option to extend all RadPdfViewer instances.
This item is declined because there are many PDF annotation types and we are going to create separate items for each type. You may follow some of the existing annotation feedback items listed below or create a new one which the Telerik team is going to approve. Here follows the list of the existing feedback items: https://feedback.telerik.com/Project/143/Feedback/Details/167439-pdfviewer-support-for-annotations-of-type-stamp https://feedback.telerik.com/Project/143/Feedback/Details/203379-pdfviewer-support-for-annotations-of-type-popup https://feedback.telerik.com/Project/143/Feedback/Details/199351-pdfviewer-support-for-freetext-annotation-type https://feedback.telerik.com/Project/143/Feedback/Details/149974-pdfviewer-highlight-annotation
The fix will be available in our official release 2015 Q2.
There will be a workaround for this issue for 2013 Q3 SP1. The CompressImages property of the ExtensibilityManager needs to be set to false, which will resolve the issue but the images won't be compressed.