JJScan lets you use your iPhone like a scanner by taking a picture of a document, cropping and adjusting it, and saving it as a PDF. You can also export the documents you scan as either PDFs or JPEGs through the app and send via email or by uploading to Dropbox or Evernote.
The first time you open JJScan, you'll see a brief tutorial that outlines the steps for scanning and saving documents. It's a useful feature, even though the process is straightforward enough and the interface of the app is quite intuitive. On the main screen, you'll see a list of categories that you can put documents in underneath a search bar. To add a new document, just tap the icon in the upper right-hand corner and choose whether you'd like to use a photo from your camera roll or take a new photo through the camera. When you have your image, you'll be taken to a screen where you can adjust the corners of the document and crop it. Then adjust the brightness, and choose whether you want the final copy to be in original, color, grayscale, or black and white. Once you're satisfied with the appearance of your document, tap the PDF icon in the upper right-hand corner of your screen to either create a new PDF with that image or add it to an existing one.
When you save your PDFs, you can also choose which category to file them under, and tapping that category on the main screen will allow you to access them later. From this list, you can also export them as many times as you'd like and whenever it's convenient for you. The app is free, and it has a lot of nice features that make it worth checking out.