You cannot keep the jar in HDFS when executing the same using hadoop command, Jar should be available in the local path

If the jar is not runnable try the following (Need to specify the package.mainclass)

hadoop jar /home/hduser/Desktop/Files/WordCount.jar wordPackage.WordCount /Projects/input /Projects/output

If the jar is runnable following can be used

hadoop jar /home/hduser/Desktop/Files/WordCount.jar /Projects/input /Projects/output

If the issue still persists, you need to rebuild this jar(WordCount.jar) in eclipse again