Here is a list of things you should do to customize document-level permissions in a subsite within SharePoint:

1. Navigate to the desired subsite, click on Settings in the top right corner.
2. Locate and click on Site Contents in the left menu.
3. Click the ellipses (…) icon to the right of the document name and choose Sharing.
4. Choose the Advanced tab.
5. From the resulting window, set the desired permissions for the document. Do this by clicking into the Readers box and typing the name of the intended user or group who needs access to the document.
6. Click OK to save the changes.