Even though the file is closed by the client in the shared folder I still can see it open when I query it with openfiles.exe

Dear Experts

In order to be more clear I would like to describe the problem step by step:

1-A client (64Bit Win8.1) puts a pdf file on a mapped shared folder on a virtual Windows Server 2012 standart.

2-Another client (64Bit win7 or 32 bit win7 or 32 bit win8.1) opens the file, checks whether it belongs to him/her.

3-She/he tries to delete the file

4-she/he can not delete it.

5-I am informed of the problem and I check the status with openfiles.exe on the server and can see that the file is still open.

6-I confirm that the file is closed by the owner and the second user.

Here is the question, why do not all the closed files on the client side not closed on the server side? How can I stop this event?

Thanking you in advance for your support.

Regards