Having 2-element Tuple as acc is not a shame. Many reduce-based solutions written by senior developers are using it. Your code is really good enough. 
However there is other way to write same code. You can use a simple pattern matching solution, for example:
defmodule Example do
# function header with default arguments
def sample(files, bookmarks \\ "", counter \\ 1)
# when done return only bookmarks string
def sample([], bookmarks, _counter), do: bookmarks
# tail recursion
def sample([file | files], acc_bookmarks, acc_counter) do
# your logic goes here, for example
{file_bookmarks, file_counter} = process_file(file)
# recursively call this for rest files with updated bookmarks and counter
sample(files, acc_bookmarks <> file_bookmarks, acc_counter + file_counter)
end
end
Personally I like separating accumulation and process logic, as in example above, which makes the code more clear.
