If your manager has set your permissions so that you have to request approval before you can send a document, you will need to request that they review your document before sending it to the client. This is nice and easy :)
When you're editing a document, in the top right corner, you'll see a button which says 'Request Approval'. Clicking this will tell you who you'll be requesting approval from and you can confirm from there.
This will put your document on your 'Pending' list while you wait for a response. Your manager can either approve or deny your request and you'll receive an email notification when there is any update made.
If your request is denied, you'll get some feedback and you'll need to re-submit the document for review once you've made the necessary changes.
If your request is approved, you'll find the document on your list of Pending documents and you can go ahead and send it from there by clicking on it and then click 'Send Document'.