Stands for "Automatic Document Feeder."
An ADF is a device that automatically feeds pages into a copier or scanner. It allows a stack of pages to be scanned or copied without user intervention. An ADF is a useful tool when scanning a long document for OCR input.
Automatic document feeders are a common feature on large, multifunction printer/scanner/copier machines meant for office use. Small standalone document scanners also often include an ADF. The mechanism for automatically feeding documents differs between models, but most can handle up to 50 printed pages.
ADF scanners differ from flatbed scanners, which require each page to be placed manually on the scanning surface. Some large multifunction devices include both an automatic document feeder and a flatbed scanner, allowing users to choose whichever scanner works best for their document.