I can not see buttons to download, edit, ... in the frontend

During installation, DOCman installs a buttons plugin. Check the plugins manager, the buttons plugin should be there and it should be published. If it isn't, something went wrong during installation, most likely incorrect folder permissions.

There are two solutions:

1. Fix the permissions and reinstall DOCman, as outlined in the readme.
2. Copy the standardbuttons.php and standardbuttons.xml files to /plugins/docman/. Insert the following statement in your database (replace '#__' with your prefix, usually 'jos_')