On rare occassions there can be problems with exported PDFs. There is an alternative method to generate PDFs, available in both Chrome and Mac OS via printing. Select File->Print and then click "Print" in the draw.io editor.
In the Chrome print dialog click "Change" by "Destination:
Then select "save as PDF" in the next dialog:
Then "save" back in the original dialog to save the PDF locally.
If you using are Mac OS, but not Chrome, printing in Safari and Firefox, for example, will show the native Mac print dialog:
In this dialog pull down the bottom-right "PDF" menu and select "save as PDF...". The next dialog will prompt for a filename and then save the PDF locally.