Depends on the platform, but I've used Readdle's Scanner Pro on iOS and it does pretty a pretty good job at auto-adjusting the contrast and geometry of scans, a

Depends on the platform, but I've used Readdle's Scanner Pro on iOS and it does pretty a pretty good job at auto-adjusting the contrast and geometry of scans, and they can be output to PDF. You can scan multiple pages into one "document" then export that document.