 but nothing shows.</span></p>
You have to create an object from the PDF class, not FPDF:
<div class="doc-source">
<pre><code>$pdf = new PDF();</code></pre>
</div>
</li>

<li id='q7'>
<p><b>7.</b> <span class='question'>I can't make line breaks work. I put \n in the string printed by MultiCell but it doesn't work.</span></p>
You have to enclose your string with double quotes, not single ones.
</li>

<li id='q8'>
<p><b>8.</b> <span class='question'>I use jQuery to generate the PDF but it doesn't show.</span>