InstaPDF is fast, clean, and easy to use. You also have access to great markup tools without having to pay for them. Sign, stamp, highlight, leave notes, and type right on the PDF without paying a penny.
What makes InstaPDF really stand out is the matching MacOS app. In order to use the mobile app or Mac app you have to sign into an InstaPDF account, but that also means everything is synced between devices. Your scans are all available on your computer right away, which is a huge selling point for this scanning app.