Dear  Susnighdha Saha,
scp uses the same public/private key system for authentication as ssh. Hence, if your private SSH key (the one you use to log in via ssh) is in a nonstandard location or/and has a nonstandard name, you have to specify it explicitly with the -i option:
$ scp -i ~/.ssh/my_private_key ptfs245h@fritz.nhr.fau.de:/home/hpc/ptfs/ptfs245h/poisson.pvd ~/Downloads/
Also, I'm not sure if it makes a difference but you should always connect to fritz.nhr.fau.de instead of explicitly to one of the frontends fritz1, fritz2, etc. And finally, your home directory is /home/hpc/ptfs/ptfs245h and not /home/ptfs245h.
Best,
Georg.