This usually happens in office setups where you need Bates numbering for a bunch of PDFs, but Acrobat isn't available. Most people try online tools first, but that gets tricky with private files since you don't really want to upload sensitive documents anywhere.
On Windows, there are a few offline tools that can do this job. One of them is PDF Champ PDF Bates Numbering Software, which lets you add Bates numbers directly on your system without going online. It works on multiple PDF files in one go, so you don't have to open each file separately and repeat the steps.
From what I've seen, the main thing people miss is checking the number placement. Some PDFs have headers or stamps already, so the number can overlap a bit. I usually test it on one or two files first before running it on the full batch.