You have multiple options when it comes to backing up PDF files:
Manual local backups: If you need to back up a single file, you can simply copy it to a different folder (preferably one that’s located on another storage device).
Automatic backup software: You can use Time Machine on Mac or File History to automatically back up your PDF files and gain the ability to recover a specific past version of any file.
Cloud backups: Services like OneDrive, iCloud, and Dropbox can automatically backup all PDF files stored in a specified folder to a secure server, allowing to access them from anywhere.