There are 2 possible ways for you to hide documents from a normal user assigned to a project.
1. To upload Project Documents and to assign the Normal user to the project without the “Can open this project” right. In this situation, the end user will only be able to access assigned tasks and tasks he/she created (if applicable) through the Home tab task lists. The project tab area will not be accessible nor will the corresponding left menu which is where Project Documents may be accessed from.
2. To assign the Normal user to the project without the “Can open this project” right and to upload Task Documents on tasks the user is neither creator of or assigned to. In this scenario, the end user is assigned to the project the same way as in the 1st scenario. The user will only be able to access the Task Documents associated with the tasks he/she is assigned to or creator of (if applicable) and that will be through the Home tab task lists.