Can users log into a protected document library by clicking on a link?
The Document Library Pro plugin comes with 3 types of access control: password, user and role protection. You can only log into a password protected document library by manually entering a password on the login page. However, user and role protected libraries work with WordPress user accounts, which gives you more flexibility.
If you want to let people click on a link to automatically unlock a protected library, then you can do this by combining Document Library Pro's user or role protection with a free plugin like Magic Login or Login Me Now. This lets customers log into their WordPress or WooCommerce account simply by clicking on a link to a secret login URL. It then logs them into their account automatically without having to enter a username or password so that they can see the protected documents which are available to their role/account.