A piece of JavaScript necessary for exporting PDFs isn't operating properly, which is preventing those PDF reports from being exported. This primarily affects previously published reports (including reports that are unpublished and republished), but brand new reports are exporting PDFs correctly.
While our developers work to fix this, you can work around this issue by enabling import settings for the report you want to export, creating a new report, and importing your old report into the new one. Publish and click the PDF Export button. You can read more about making reports importable here:
https://raven.zendesk.com/hc/en-us/articles/203231670-How-do-I-make-my-WYSIWYG-reports-into-importable-templates-