[Koha] How to hide Subfield in OPAC

[Ketut Gunawan]

Dear all,
I am new to Koha so I need helps. I have a subfield 856$u containing PDF files in hundred of records. I want to hide it in OPAC so that public visitors cannot see "Online resources: Click here to access online", but for those who log in (students)  can see and click such a link.

I have tried to read the manual and search the forum, but I can find it. Any help would be much appreciated.

Best Regards,
K. Gunawan
SocPolSci Faculty, Mulawarman University
Indonesia