When it comes to productivity apps, there are very few that have lasted me for an entire iPhone generation before being replaced my something better. Extend that filter out to two or three years and it's a very short list. TurboScan is one of those apps, and it's remained the best document-scanning app since I first downloaded it in early 2010.
TurboScan eliminates the need for a physical scanner to create digital documents, but its list of features is what really makes it irreplaceable for me. For starters, the scanning functionality itself is amazingly fast and easy to use, requiring just a single photo of the document. The app automatically detects the edges of the document and crops it for you, or you can choose to specify the cropping areas yourself.
You can adjust the contrast of the scan to make the copied text or images darker or lighter, and you can even choose to do a color scan or simply save the photo of the document outright (which kind of defeats the point of the app, but it's an option).
The "SureScan" mode takes multiple photos of your document to ensure a clean sample is obtained, but I've never had an issue with just using a single photo. Multiple pages can be added on the fly, and documents are saved in groups, which is super convenient for forwarding.
Speaking of forwarding, you can use the in-app forwarding features to send your documents out in multiple formats, including .jpg and .png image files, as well as PDF. The "Email to Myself" feature automatically forwards your scanned items to your own email address, which is the quickest way to get the documents from your phone to your computer.
The app costs a totally reasonable US$2.99, and for an app that I've gotten four years of use out of, that's quite the bargain. If you still find yourself needing to scan documents for any reason, it'll pay for itself before you know it.