How to execute a function at start in Emacs

I want start following function (from Replace one space by two after sentences in Emacs) to run when I right click a file in file-manager to open the document with Emacs.

(defun space12 ()
  (interactive)
  (save-excursion
    (goto-char (point-min))
    (while (re-search-forward "\\. \\([^ ]\\)" nil t)
      (replace-match ".  \\1" t))))

This converts ". " to ". " without increasing spaces of existing two space occurrences.

How can this be done. I thought of adding (space12) to init.el but it seems to be loaded before the document is loaded.

Sample input:

This is for test. This is second line with only one space at start.  This is third line which already has 2 spaces before it. End of document.

Try adding this to your init.el:

(add-hook 'find-file-hook 'space12)

However this will run your function on every file you open. Is that what you want?