we are trying to find a file server for us. we look for the best practices of the establishment of an efficient and secure server. Therefore, my question is: What is recommended: server as a PC or server as a hard drive disk.
In theory a NAS (which I assume is what you mean by "server as hard disk") is no different from a traditional file server in it's operation. They are both computers running an OS with a bunch of hard disks. There is no underlying reason why one or the other might be more secure.
In practice, it all depends on you. A NAS comes pre-built, whereas you build a traditional filer yourself. So, if you are an expert, you can use the finer control you have over a traditional server to harden it against attack. However, if you are not an expert, it's much easier to make mistakes in setting it up and leave holes open for attack. A NAS is going to fall somewhere in the middle.
So, if you don't know what you are doing, a NAS is likely to be stronger out of the box.
If you do know what you are doing, then usually the decision of "NAS or traditional" is going to be determined by other factors, such as cost, reliability, capacity, ease of use, etc. etc. and you'll have to harden whatever you get.
If security is absolutely your number one priority and you do not care about anything else then you're probably going to want to build a traditional server yourself and hand craft every detail for maximum safety. But that is a very unusual requirement.