My accountant sent me my tax returns as password protected PDF files. There isn’t any way to get them out of Adobe Acrobat Reader without the password. There are lots of on-line solutions, but that means giving someone else access to your data. There are also programs that you can download, but it’s hard to know if you can trust them. Instead just drop your PDF onto Safari. Enter the password, then use File –> Print to print to PDF.
If you want to add a new password, open the file in Preview and then use the Export as PDF command. Click on the Show Details button and check the box for encryption. You can add your own password here. The encrypted files will open in Reader and Preview.