On my site I have installed K2 and Docman. After I inserted all documents in Docman I created some on K2 items and I want to display Docman documents only to registered users.
Anybody have a solution?

Thanks

These are two seperate extensions. Please be more specific. ie: have you embed the docman into k2, or are you asking a docman question. The same approach may work for hiding things from non registared users.

Both.
Embedding Docman in K2.
I have to embed the docman into k2, my K2 items must have an area visible only to registered users where to show DocMan documents linked to k2 items. I can insert Docman documents with the doclink button in to K2 items but these link aren't hidden to non registered users.
Docman question (but I know this isn't the place)
I can create a Docman module visibile only to registered users but I don't know how to link a Docman documents with the k2 items in order to avoid to create a module and a menù item for every k2 item.
What do you recommend?

Thanks

If you have already embeded the appropriate docman document into your k2 item, is there some relationship between the k2 itemid and the docman plugin that your using.
I can most likely create a k2 override that will insert the proper tag for you in the k2 item or category display or both automatically and also be hidden unless the user is registared.