Export as PDF in landscape mode

In the meantime, as a workaround, content that should be in landscape mode can probably be 90° rotated using CSS in a media print declaration. This declaration can be put in the PDF template or elsewhere. You probably want to put page breaks around this content.

For instance (this is untested though):

@media print {
    .landscape-content {
        break-before: page;
        break-after: page;
        transform: rotate(90deg); /* or rotate: -90deg */
    }
}

And then you can put your content inside a div of class landscape-content:

In HTML:

<div class="landscape-content">
    <p>content to be rotated.</p>
</div>

In XWiki syntax:

(% class="landscape-content" %)
Content to be rotated
(%%)

The drawback is that the concerned pages will look rotated on screen, but most PDF viewers can rotate individual pages and even save the result.