Complete datasheet in PDF form, 1.23 MB, for the tiny26, which corresponds to the Atmel AVR ATtiny26 8-bit microcontroller. No revision is given in the name. Such a document details the pinout, ADC and timer peripherals, clock and fuse settings, absolute maximum ratings and the in-system programming procedure. A technician would open it in any PDF reader to identify supply and ground pins, locate the reset and programming lines for an ISP connection, and confirm operating voltage when diagnosing a dead or misbehaving controller.
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.