I am trying to install NIFI on top of HDP cluster which i already have but i didn't find a clear documentation for that
You should be able to use the HDF management pack (mpack) to install NiFi on top of your existing HDP cluster.
See the following documentation for details: https://docs.hortonworks.com/HDPDocuments/HDF3/HDF-18.104.22.168/bk_installing-hdf-on-hdp/content/ch_insta...
For those who will come next, you may face some trouble in installing nifi on HDP 2.6 even if you have mpack installed, I have some trick but it's simpler to do as follow.
You can try the new version hdp 3.0 sandbox it's more stable. Download it in the cloudera website and install mpack following the documentation https://docs.cloudera.com/HDPDocuments/HDF3/HDF-22.214.171.124/installing-hdf-and-hdp/content/installing_th..., then you will easily add nifi 1.9.0 as service.