I have got this somehow to work:
<?php
$text = get_field('dokumente_datei');
$stripped_text = str_replace(".pdf","-pdf.jpg","$text");
?>
<img src=<?php echo $stripped_text; ?>" width="100%">
But it returns the full-size image and I need a smaller one for the post loop. As the smaller ones are named differently (pixel size added to the image names), it does not work either as I try to get it to work….
