How you go about this will depend on the software you're using to create the file in the first place. Some applications have password protection features built in, while in other cases you'll need ...

Prior to Big Sur, locked files would have a lock image overlaid on its icon in the Finder. Big Sur only shows that lock overlay in the Get Info dialog box.

You can embed a PDF file into a cell of an Excel worksheet and then use Excel's security features to lock the object. This allows you to take advantage of both technologies.