Rather than downloading a single .h file, you should install the complete library to ensure all dependencies (like Adafruit_BusIO ) are met. Open the Arduino IDE .
The adafruit mcp23s08.h file is a core component of the , which provides a unified interface for the MCP23xxx family of I/O expanders. This header file specifically manages the MCP23S08 , an 8-bit SPI-based port expander that allows you to add extra GPIO pins to your microcontroller using only a few SPI lines. How to Download and Install adafruit mcp23s08.h download