Existing cameraphone-based interactive paper systems fall short of the flexibility of GUIs, partly due to their deficient fine-grained interactions, limited interaction styles and inadequate targeted document types. We present PACER, a platform for applications to interact with document details (e.g. individual words, East Asian characters, math symbols, music notes, and user-specified arbitrary image regions) of generic paper documents through a camera phone. With a see-through phone interface, a user can discover symbol recurrences in a document by pointing the phone’s crosshair to a symbol within a printout. The user can also continuously move the phone over a printout for gestures to copy and email an arbitrary region, or play music notes on the printout.