A 761 KB PDF titled 28256a, filed as a schematic but most likely a fuller datasheet edition for a 28-series 256 Kbit parallel EEPROM, the a suffix suggesting a revision or variant. No vendor is named. Technicians use it to look up pinout, page write and software data protection sequences, and timing before dumping or programming the chip. It is much larger than the plain 28256 file, so it probably carries more detail. Verify the chip marking and back up its contents before writing.
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.