The https://docs.microsoft.com/en-us/javascript/api/office/office.document?view=word-js-1.3#getFileAsync_fileType__callback_-API already supports PDF export. Unfortunately, the file format is PDF 1.8...
We also support this idea and we would love to extend the options to cover all the PDF export attributes: pages to include, properties, image quality, etc. Basically, extending the API with similar options as the old Saveas2 method in VBA, but for the JS API. This should include the export differences of PDF options for Word, Excel, and PowerPoint.
Thank you!
Word:
PowerPoint:
Excel: