In EFS, when you click on an uploaded document, it would bring it up in a separate window or tab of the default browser using whatever PDF plugin you had. In Patent Center, it instead brings it up in a pop-up that overlays part of the submission webpage. The old way was better for at least the following reasons:
a) You can only bring up one document at a time in the new system, forcing the user to wait for each document to load before reviewing it. In the old system, you could hold CTRL down and click each document link and the browser would load all of the documents in parallel, displaying them in separate tabs of your browser (this is in Chrome, at least). You could thus review one document while the others finished downloading.
b) Users could control the size of the window that the documents were displayed in. In the new system, users cannot resize the pop-up window. This makes it difficult to review documents, as you have to adjust the PDF scale so that it fills the pop-up window, and then potentially adjust the browser scale so that the scaled PDF is legible, and this then causes the underlying GUI to scale as well. You shouldn't need to re-scale your GUI to view a PDF that could be popped up in a separate window so that it is independently viewable.
I would recommend reverting to the old system.