We have a good news for all of you who have experienced problems with the memory consumption of RadPdfViewer. We have made significant optimization of what is cached for each page when scrolling in a document.
Please have in mind that when using big images in a document the optimization may seem not so significant. This is because there is additional opportunity to further optimization of the caching mechanism for the image sources.
I have logged this additional opportunity as a different feature request. Here is a link if you would like to subscribe for status updates of that item to: Optimize the image source cache - http://feedback.telerik.com/Project/143/Feedback/Details/189305-optimize-the-image-source-cache