Download, install, and run the SysInfo PDF Protect or Unprotect Tool as an administrator.

Select the Protect PDF File option to apply a password to an unprotected PDF file. Hit Next.

Define the location for a Single PDF file or Multiple PDF files and click on the Next button.

The PDF file description will be shown in the preview panel, check all the details, and press Next.

After that, choose the encryption level for the output PDF file and enter the User and Owner Password.

Enter the User Password and Owner Password that you want to set to protect the file.

Apply other settings based on your requirements like allow Low-Resolution Printing, allow changing the document, and more. Hit Next.

Now, you can check the description for the output PDF file. It includes details like Author, Title, Subject, Keyword, Producer, and Creator.

You can choose to enter a unique name for your resultant file in the Output PDF File Name column.

Further, click the Browse icon and select a location where you want to save your protected PDF.

Finally, click on the Protect PDF option to secure your PDF File.

Lastly, click on OK when the saving process is over and your PDF file is password-protected.