Only the user that shared the documents con edit them. The recipient cannot.
In order to edit shared documents, go to Shared with others > User’s email folder, select one or multiple documents, right click and select Sharing > Edit Share option.
Edit Share window is opened showing the permissions and expiration date initially set up.
The user is able to change all settings.
Permissions when editing are the same permissions allowed when sharing. For example, if the user originally was able to select Download PDF, he should be able to select the same permission when editing shared documents.