In Documents tab of a deal there are different document actions you can select. Lock file is a feature that allows you to lock documents from being downloaded, shared or amended by other users.
To start, open a Deal and click on Documents tab:
In Documents tab, click on Lock file icon on right hand side to lock the file:
After Lock file option was applied all other Document action buttons will disappear and all other action with document won't be possible:
Documents can be easily unlocked by clicking on the red icon that is visible to admin user. In case that non-admin users open a deal, document will present blank to them - they won't see the unlock icon or be able to press it: