Letters, receipts, contracts etc. can be scanned using document scanning apps that use mobile's camera.
Keep them safe & store it in cloud (on internet)
Top document scanning apps
1. Dropbox Business - Very popular, 500 million users. Scan & save on cloud.
2. Scanbot - Inbuilt decision capability to scan document perfectly by straightening & cropping.
3. Genius Scan - Scan paper document & convert it into JPEG or PDF files. Enhances images.
4. Abbyss FineScanner - Uses optical character recognition to scan text in 193 languages. Convert your scanned document into MS word format.