To customize the permissions for a specific document in SharePoint, follow these steps:

1. Open Document Library where the document resides.
2. Click on Gear icon > Library Settings.
3. Click on Permissions for this document library.
4. Click on Stop Inheriting Permissions.
5. From the ribbon select the document you want to customize permissions
6. From the ribbon select Manage Permissions.
7. Grant or deny permission for users as needed.
8. Save your changes.