Suppose if I uploaded to my Github repository a
.pdf of a technical book I've written and want to share. According to
CC BY-NC-ND 4.0, if someone were to fork the repository, they would still be bound by the
CC BY-NC-ND 4.0 to not modify (create a derivative) and redistribute that derivative, correct?
Yes, that is correct. Forking a repository does not magically erase the license that you have attached to the work.
If someone where to change the format, for example, from
.txt (which I don't mind), because of the
CC BY-NC-ND 4.0 licence, they would have to leave the book text, they can't modify it, correct?
Yes, that is correct.
Furthermore, although changes in the format are allowed by the license (in section 2(a)(4), entitled "Media and formats; technical modifications allowed."), I believe that a conversion from PDF to plain text is not a clear-cut case of such an allowed format change. I believe that this change affects the nature of the work in such a way that it creates an Adapted Material, which the creator wouldn't be allowed to distribute due to the CC BY-NC-ND license. That does not prevent you from giving separate, additional, permissions to people who make such radical conversions.