A 128 KB PDF named 25C256, which matches the pattern of a serial memory chip part number, so this is most likely a chip datasheet placed in the schematic category. No brand, host device or revision is stated. The datasheet is handy for confirming pin functions, supply voltage and package before clipping a programmer onto the chip to read or rewrite it. Keep a backup of the original contents before writing, and check that the image size you intend to flash matches the capacity of the installed part.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @cyberlanelectonik. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.