The comparatives between Web Designer Premium and Designer Pro https://www.xara.com/eu/designer-pro/compare/ (if this is up to-date) suggest you do not have an PDF export option in Web Designer Premium.

With Pro, the Viewer Options, assuming Export is a Transitional/ Supersite provides the additional PDF checkbox. If you export as a Website, these options are not available.

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I can't tell whether the WDP exclusion is intended in the new releases.

Gary