A schematic is the circuit diagram of the main board: power rails, power-up sequence, component values and net names. It is the reference for measuring voltages and tracing a fault.

Tools to open these files

Format	What it is	Open with
.pdf	PDF schematic	any PDF reader; Sumatra PDF and Foxit handle large schematics well
.rar	RAR archive	7-Zip or WinRAR
.zip	ZIP archive	7-Zip, WinRAR or the built-in Windows extractor