It is preferable to use blank characters instead of tabs to indent the source code: the interpretation of the actual width of a tab is up to the viewer program.
Please use blank characters when writing code to be included in GNU PDF software.
If you use Emacs you can tell it to insert spaces instead of tabs including:
(setq-default indent-tabs-mode nil)
in your .emacs.
If you use GNU indent to indent your sources you can use the
--nut option:
$ indent --nut [rest-of-parameters] [source-files]