Not natively with Intuit Link.  Your have two alternatives:

1. Change user's access to each client within Intuit Link by clicking on the gear icon next to your firm name in the top-right corner, selecting Manage Users, clicking on the name of a particular user, clicking on the Client Access tab, and unchecking the client(s) you don't want that user to access.   (This has to be done one-by-one and is not dynamic, which means you will need to add access for each new client individually); and
2. Password-protect the PDF file before uploading that to Intuit Link.

The return you upload to Intuit Link is no different from anything else residing there.  Unless you control user access to each particular client in Intuit Link, every staff will have full access to everything in Intuit Link.