Open the PDF in Adobe Acrobat
Go to File
Select Properties
Open the Security tab
Choose Password Security from the Security Method menu
Check Require a password to open the document
Enter the password
Confirm the password
Click OK
Save the PDF
Open the PDF in Microsoft Word
Go to File
Select Save As
Choose PDF as the file type
Click Options
Enable Encrypt the document with a password
Enter the password
Confirm the password
Save the file
Use an online PDF password tool
Upload the PDF
Select the password protection option
Enter the password
Apply the protection
Download the protected PDF
Use a PDF editor or printer tool
Open the PDF
Select the password protection or security option
Set the password
Save the protected PDF
On Mac, open the PDF in Preview
Go to File
Select Export
Check Encrypt
Enter the password
Confirm the password
Save the PDF