Looks like the problem lies in this.
This:
<?php if( have_rows('newsletter_files_section', $stronaPdfy) ): the_row(); ?>
<?php while( have_rows('newsletter_files_section', $stronaPdfy) ): the_row(); ?>
Should be like this:
<?php if( have_rows('newsletter_files_section') ): ?>
<?php while( have_rows('newsletter_files_section') ): the_row(); ?>