Short Atmel document tied to the AT89S8253 microcontroller, marked AN in the name, which most likely indicates an application note rather than the full datasheet. At 48.7 KB it probably focuses on a single topic such as programming, migration from an earlier part or a usage example; open it to confirm. It complements the main datasheet when setting up in-system programming or porting code on 8051-family boards. Keep the full datasheet at hand for register and pinout details, and verify the chip marking first.
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 @datasheet. 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.