Searchable PDFs created from scanned originals

From Termhotel.com

See also: digitise vs digitalise

Paper dictionaries are useful, but even more so after having been digitised and converted into searchable PDFs. These PDFs will typically contain both the original scanned image plus a separate text layer produced using optical character recognition (OCR). These files can then be indexed in a desktop search program such as dtSearch for quick and easy access to the data.

I have all my valuable reference books processed in this way by a professional scanning company called Digitisemybooks.