I have the same problem as morca even with cron set up and running correctly. For me the problem was introduced with version 1.2. Before I received the scheduled PDF reports.
Update: I had a look at the output of the archive.sh script and noticed that scheduled task section shows “No data available” even for days when a report should have been scheduled.
The pdf reports are stored correctly in table “piwik_pdf” as far as I can see. Column “ts_last_sent” shows the dates I last received the reports automatically (Before upgrading to 1.2).
Strange thing is, I setted up a fresh daily PDF report yesterday and received it like planned this morning. The script output also showed the processing of this task correctly.
To sum that, for me, the problem only exists for scheduled pdf reports that were created in versions earlier than 1.2.1. Those stopped working after upgrading.
I will now recreate all PDF report schedules and look what happens.