If you create a Custom HTML macro, like a drop down with multiple options, it will work fine in the Confluence page:

However if you use the "Export to PDF" function it will show you all the drop down options instead of the selected one:

Steps to reproduce:

1. Go to Confluence Admin > Manage Add-ons.
2. Enable all the modules of the Confluence HTML system plugin.
3. Create a page with a Custom HTML macro with below code:

   <select>
   <option value="1a">Option A</option>
   <option value="2a">Option B</option>
   <option value="3a">Option C</option>
   <option value="4a">Option D</option>
   </select>
   

4. Save it.
5. Now Click in Tools > "Export to PDF".
6. You will see that all the options will show up in the pdf export instead the selected or the first one.