Based on your last comment
I create a webpart to show the
Document List on a certain page, and
then I applied a filter on a column
content to show only certain
documents. But in that way the folder
structure is lost and all documents
are shown flat.
I think I can answer your your question now.
If you create a new view and select in the filter section
And then add the additional condition you wish to apply to the documents.
In this way folders will always be displayed but their contents will be filtered based on the additional constraint.
This will cause folders to be displayed whether or not they contain any children matching the additional constraint but you will be able to navigate the hierarchy.