Datasheet for the older ATmega161 AVR microcontroller, a 1.65 MB PDF document. It details pin functions, the memory map, peripherals such as timers and UARTs, programming modes and electrical characteristics. Repair technicians use it when servicing legacy equipment built around this part, to check supply pins, oscillator wiring and reset circuitry or to prepare a programmer for reading and rewriting the flash. Read the original contents first and keep a backup, because lock bits may prevent a later read.
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.