Article: Go to https://smallpdf.com/protect-pdf/ in your web browser. This website will allow you to add to your PDF a password that must be entered before the PDF can be opened. It's a link in the middle of the page. Doing so will prompt your computer's File Explorer (Windows) or Finder (Mac) window to open. Go to the location of the PDF that you want to password-protect, then click the PDF. It's in the bottom-right corner of the window. This will upload the PDF to the SmallPDF website. Type the password that you want to use into the "Choose your password" text field, then re-enter the password in the "Repeat your password" text field below it. Your passwords must match each other in order for you to be able to continue. It's a red button below the password text fields. The password will be applied to your PDF. You'll see this button on the left side of the page. Clicking it prompts the PDF file to download into your computer's default downloads folder, though you may first have to select a save location and click Save depending on your browser.

What is a summary?
Open SmallPDF's protection page. Click Choose file. Select a PDF. Click Open. Enter a password. Click ENCRYPT PDF . Click Download File Now.