adf I think stands for Amiga Data File. Might be wrong, but if I remember correctly, adf files can only be used by an emulator.
Amiga Disk File.
Anyway it's image of amiga floppy disk, two sides, 80 tracks per side, 11 blocks per track, 512 bytes per block. That's total 901120 bytes, or 880 kilobytes.
Now, it's quite apparent Deluded is using an emulator (AmigaForever) and don't want to write the adf to disk (and they couldn't anyway, without a real amiga).
But fear not, AmigaForever can use the 'adf' files directly! There is no need to extract anything. To learn how to use the files, look into
AmigaForever FAQ: Using ADF, ADZ, FDI and DMS Files.
In general
AmigaForever FAQ is worth a read.
