Perhaps you should also mention other categories of scanning apps (Or have separate HowToGeek articles on them). The two I have in mind are:
1) Photo scanning apps: These are designed to scan photos rather than documents, and output JPEG rather than PDF files. More importantly, their editing functions are meant for editing photos (filters, captions, etc.) My favorite is Pic Scanner (www.appinitio.com/picscanner), because it can do simultaneous scanning of up to 3 photos, plus their automatic and accurate cropping. It is a free app for iPhone and iPad, and the website says Android version is coming.
2) QR Code scanning apps: These can use the smart phone camera to scan QR or bar codes, then decode and use the data to automatically search the internet for relevant underlying information. My favorite is Discover (http://www.digimarc.com/discover). It can scan barcodes, QR codes, as well as images, audio and video. It is free and available for iOS and Android.