How do I decode a list of base64-encoded file names?

Just add the base64 encoding of newline (Cg==) after each file name and pipe the whole thing to base64 -d:

find . -name "*_*" -printf "%f\n" |
  sed -n 's/_....-..-..\.pdf$/Cg==/p' |
  base64 -d

With your approach, that would have to be something like:

find . -name "*_*" -printf "%f\0" |
  sed -zn 's/_....-..-..\.pdf$//p' |
  xargs -r0 sh -c '
    for i do
      echo "$i" | base64 -d
    done' sh

as you need a shell to create those pipelines. But that would mean running several commands per file which would be quite inefficient.

One trick is to encode \n in base64 ... so it becomes Cg== this you can append to the printf-command. A '\' cannot be in a filename. So in the end you can sed it back

find . -name "*_*" -printf "%f\0Cg==" | sed 's/_....-..-..\.pdf//g' | xargs -0 -i echo "{}" | base64 -d | sed 's/\\n/\n/g'