The Missing Documents report lists every driver and unit that doesn't have a document you've marked as required.
Go to Reports.
Open Missing Documents.
Use the filters to narrow the list, and sort by type or tags.
Click Export CSV if you need a copy.
Which documents are required? The report uses your Doc Type settings. Go to Settings > Doc Types and set Required to Yes for each type every driver or unit must have. You can also see missing documents on the Dashboard in the Missing Docs and Live View tabs, or get a daily email with Required Docs in Settings > Notifications.