To start to be able to send PDF reports to users, this is an option you have to pay for. The users you send the reports to have to be in an AD or Custom User having an email address. It is only a report and this has to be built into the QVW. You have to select the report ou want to send in the drop down on the distribution page.
I got the same error and after research about it I got to this conclusion:
When we have a task which send a PDF by mail we can not have any hidden object in our report, because for any reason Qlikview lose the reference to these objects and QV doesn't know how to make the report.
I hope it helps