Hi, I am a starter and a student. I am looking for a PREEMPTIVE RTOS for PIC18Fxx8 and CAN.Could you please suggest me any available options. I am currently using PICOS18 but I am finding it difficult to implement because I am finding the documentation difficult to understand.

FreeRTOS.org runs on the PIC18, but this is really at the very low end of where it is appropriate to use a kernel.  You can look at the UART examples to see how to integrate in the CAN drivers. Regards.