Open the PDF in Adobe Acrobat Pro
Click the Tools tab
Select Protect
Choose Encrypt
Select Encrypt with Password
Enter a password under Document Open Password (if you want to require a password to open the PDF)
(Optional) Enter a password under Permissions Password (if you want to restrict printing/copying/editing)
Select an encryption method (default is usually fine)
Click OK
Re-enter the password to confirm
Save the PDF (Save As to create a new protected copy)
Use Adobe Acrobat Reader (if you only need to secure with a password and your version supports it)
Open the PDF
Go to File
Select Properties (or Protect)
Choose Security/Password options
Set the password requirements
Save the file
Use macOS Preview (if you only need basic password protection)
Open the PDF in Preview
Click File
Select Export
Check Encrypt
Enter a password
Re-enter to confirm
Choose Save
Use Microsoft Edge (if it supports PDF encryption in your version)
Open the PDF in Edge
Look for a Print/Save-as- PDF security option (if available)
Set password protection
Save the encrypted PDF
Use an online PDF password tool (only if you accept uploading files to a third-party service)
Upload the PDF
Choose password protection
Set open/permission passwords
Download the secured PDF
Remove the uploaded file from the service if the site provides a delete option
Verify the result
Close and re-open the secured PDF
Confirm the open password requirement (if set)
Confirm restrictions (copy/print/edit) match your permissions settings