Datasheet for the Microchip PIC16F818 and PIC16F819, 18-pin flash microcontrollers, supplied as a 2.97 MB PDF. The two parts differ mainly in program memory size and share pinout and peripherals, including an internal oscillator and analog inputs. A repair technician uses the document to confirm power, reset and programming pins, review configuration words and check input thresholds. When reflashing, use a programmer that supports the exact part and save the original contents first if readable.
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.