Autogenerated PDF Thumbnail using PDFThumb in a Matrix Field
You mixed in some code from the documentation resulting in a wrong variable name..
{% set pdfBlocks = entry.inhaltMatrix.all() %}
    {% if pdfBlocks|length %}
        {% for block in pdfBlocks %}
            {% if block.type == &quot;pdf&quot; %}
                {% if block.pdfFile %}
                    {% set rel = block.pdfFile.one() %}
                    {% if rel %}
                        &lt;a href=&quot;{{ rel.url }}&quot;&gt;{{ rel.title }}&lt;/a&gt;
                        &lt;img src=&quot;{{ craft.PDFThumb.thumbnail(rel) }}&quot;&gt;
               
                    {% endif %}
                {% endif %}
            {% endif %} 
       {% endfor %}
{% endif %}

Also be sure to go to the plugins settings and set the appropriate assets paths.