each_with_index_do starting at 1 for index

Unless you’re using an older Ruby like 1.8 (I think this was added in 1.9 but I’m not sure), you can use each.with_index(1) to get a 1-based enumerator:

In your case it would be like this:

<% @document.data.length.each.with_index(1) do |element, index| %>
  ...
<% end %>