Android: If your phone has a good camera on it, why would you need a separate app to grab a head-on shot of paper? Handy Scanner straightens and fixes perspectives, enhances readability, makes easy work of multi-page documents, and pushes PDFs quickly to wherever you'd like.
Perhaps the most helpful feature of Handy Scanner is its perspective tool, which asks you to quickly outline the paper you just snapped, and then analyses the image to flatten it out. After that, you get a handful of tools to make the text more readable, to convert the image to grayscale, and make other fixes.
Finally, you can either add more pages, or if you're done, send a full PDF out through whatever services are in your Android "Share" menu -- Dropbox, Google Docs, whatever you like. You can also simply save the PDF to your Android storage space.
Handy Scanner is available as a free download, with a few token limitations -- number of pages per document and watermarks among them. The Pro version is $3.79.
Handy Scanner Free PDF Creator [Google Play]