You will want to ensure that the space inherits the same permission as the parent space. This can be view by going to Manage > Permissions, as shown below:
This will take you to the admin console where you can Check Someone's Permission for that particular space:
If you are able to verify that the user should have access to view that space as outlined above and is still unable to view the document, I would recommend creating a case in your Private Support group. This will allow our Support to further review the document and user.
Hope this helps.
Great! thank you!