Warning: Cannot modify header information

This looks like a very useful plugin. However, when attempting to view an uploaded pdf document I get the error below. Tried from front and back end.

Warning: Cannot modify header information - headers already sent by (output started at [..]/wp-content/plugins/board-document-manager-from-chuhpl/showMe.php:26) in [..]/wp-content/plugins/board-document-manager-from-chuhpl/showMe.php on line 35