Open the PDF in Adobe Acrobat
Go to File
Select Properties
Open the Security tab
Choose Password Security
Check Require a password to open the document
Enter the password
Confirm the password
Save the PDF
Use Microsoft Word
Open the document in Word
Go to File
Select Save As
Choose PDF as the file type
Click Options
Set a password if available through your PDF tool or add-in
Save the file
Use a PDF editor online
Upload the PDF
Select the password protection option
Enter the password
Apply the changes
Download the protected PDF
Use Preview on Mac
Open the PDF in Preview
Go to File
Select Export
Check Encrypt
Enter the password
Save the PDF
Use a printer-to-PDF tool with security settings
Open the PDF in the tool
Select print or export settings
Enable password protection
Set the password
Save the new PDF
Use a command-line tool like qpdf
Install qpdf
Run the password-protection command
Set the user password
Save the encrypted PDF