Load any PDF file. Change the viewing settings with the context menu. Scroll the document. Now go to any other URL. Go back. None of the settings you made to the PDF view are remembered. This can be frustrating, and is not consistent with how HTML or plain text pages behave.
This is definitely a UI bug with the current implemenation of WebPDFView and the way it integrates with WebKit.
I'm tackling this one.
This has been fixed by the patch in 4091.