Are you trying to download the PDF from within Dashboard or from within Management > Reporting? If the latter then it's possible the user doesn't have access to the data used to generate the report. According to the built-in-roles docs:
reporting_user
Grants the specific privileges required for users of X-Pack reporting other than those required to use Kibana. This role grants access to the reporting indices. Reporting users should also be assigned the kibana_user role and a role that grants them access to the data that will be used to generate reports with.
Let me know if this helps or if the problem is something else.
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