Instruction set reference in PDF format, 758 KB; the name alone does not specify the architecture, though it sits alongside AVR documents and most likely is the Atmel 8-bit AVR instruction set manual. Such a manual lists every opcode with its operands, flags affected, cycle counts and encoding. Technicians and firmware tinkerers use it when disassembling a dumped flash image, patching code or understanding timing loops. Verify the title page matches the processor family you are working on before relying on it.
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.