First check the export with different browser and different pdf viewers. Always blank?
Another reason why no text is shown can be corrupted fontfiles.
You find them under /admin/classes/tcpdf/fonts.
Best thing would be to update to the newest version via comfortupdate and try to export results again.
If still buggy than reupload the tcpdf/fonts from the newest limesurvey release to see if the fonts are the reason.
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