The header file is not a standalone "downloadable" library file in the traditional sense. It is the [5.5, 5.6].
The xc.h file is a provided by the MPLAB XC Compilers . Instead of manually including specific files for every different microcontroller (like pic16f877a.h ), you include xc.h , and the compiler automatically pulls in the correct register definitions based on the device you selected in your project settings. Where to Download the Library xc.h library download
Based on the device you select in your project settings, xc.h automatically "vectors" or points to the correct device-specific header. The header file is not a standalone "downloadable"