1. Login to Plesk control panel.
2. Under Websites & Domains tab click Password-Protected Directories
3. Click Add Protected Directory
4. Specify the directory, located inside Document Root ('httpdocs' folder on our servers), that should be secured. In this case, we are restricting an access to the /httpdocs/pdf folder (example.com/pdf). Click OK.
5. Now, click on the directory name. In this example, /pdf
6. Click Add a User to create a user that will be used to access the password protected directory. Click OK.
7. Open the protected URL in a web-browser (in this example, example.com/pdf) and log in using the user account you just created.