Andy Lester's ack will do that quite nicely.

To search files NOT named ".lst" for the string ".txt":

ack "--ignore-file=match:\.lst" "\.txt"
[download]

Or in your case something like

ack "--ignore-file=match:<user stuff here>_ABC_.*\.pdf" "\.txt"
[download]

Yes, it's coded in perl.

Yes, I'm on Windows. For *nix use 's in place of "s.