PDFMitosis

PDFMitosis takes your PDF and creates a new document in which each page is duplicated in order. Your original document, with pages 1, 2, 3 etc., remains untouched.

Your new document now has pages 1, 1, 2, 2, 3, 3 etc. You are then free to view your document in "Two-Up" format. Then, you can let loose your inner annotator on one sheet while leaving its twin pristine next to it.