What you might be looking for here is the elseif statement…
<?php if ( in_category('123') ) { ?>
<div class="pdfbox">
<h1><a href="https://wordpress.stackexchange.com/questions/116680/<?php the_permalink(); ?>"><?php the_title(); ?></a></h1>
<p> </p>
<a href="https://wordpress.stackexchange.com/questions/116680/<?php the_permalink(); ?>"><?php the_post_thumbnail('pdfs'); ?></a>
</div>
<?php } elseif ( in_category('125') ) { ?>
<div class="pdfbox">
<h1><a href="https://wordpress.stackexchange.com/questions/116680/<?php the_permalink(); ?>"><?php the_title(); ?></a></h1>
<p> </p>
<a href="https://wordpress.stackexchange.com/questions/116680/<?php the_permalink(); ?>"><?php the_post_thumbnail('pdfs'); ?></a>
</div>
<?php } ?> { ?>
Hope that helps.