If your client gets in touch to say that they can't find a document to review, then the first think you'll need to check is your Folder Templates settings. 

Head to Settings - Portal - Folder templates

From here, review the folders paying close attention to 'READ' which appears in Green. So, locate the area in question and ensure that READ has been enabled, which means your client can then see the document in question. 

If it hasn't been switched on, select 'change sharing options' 

Client can't find document

Client can't see