Saw someone who's a web developer explain it. It's an error from the department tasked with transcribing/archiving them. Something about a line-break being misread and replacing characters when it shouldn't. Kind of like when you see "&amp&" and junk like that.

I suspect they came up with a process for doing the conversions / redactions that worked for the documents they tested it with, and then applied the process to documents in a slightly different format and just didn't notice. The Verge looked into it and couldn't get a more specific answer than this:

With MIME, the “=” is used to signal either that a string of text should be broken for transmission and rejoined — a “soft line break” — or, when followed by two other characters, that it should be converted to a particular non-ASCII mark.

it doesn’t fully explain why the “=” sometimes replaces letters, like the “J” in “Jeffrey.” No one I spoke to could definitively answer this question, except to say that email is hard and converting it to PDF is harder, and the DoJ was converting a lot of documents in a hurry.

https://www.theverge.com/policy/879016/epstein-files-emails-text-errors-encoding

Mails are often encoded in the "quoted-printable" format. It uses = as an escape character and it is used in different ways. Besides soft line breaks also some special character are encoded using a equal sign, followed by hexadecimal characters. The wiki includes some examples: https://en.wikipedia.org/wiki/Quoted-printable

This might explain the replacement.