DNG is an open-source raster image format developed by Adobe Systems and an extension of the TIFF 6.0 cross-platform universal format. The name of the format is an abbreviation of Digital NeGative Specification. The novelty was announced by the developer in 2004 as a standard format for RAW files. It is large in size because it contains not only the main image of high quality, but also metadata, as well as several images for preview (lower quality and resolution). Digital photos in this format are stored in uncompressed form.
PDF is short for Portable Document Format, which can be translated from English as “Portable Document Format”. It was developed by Adobe Systems for use by US federal authorities as a tool for storing working documents. This is a universal cross-platform format, which is now standard for electronic documents. It serves to convert text files (including photographs or other images) into electronic documents without any loss. To read PDF files you need special programs - Adobe (Acrobat) Reader, PDF-Viewer and others.