How do I set the margins on a PDF print out of a CiviReport?
As of CiviCRM 5.3 the margins on a PDF print of a CiviReport take their margins from the default PDF page format. This can be administered at:

Choose: Administer >> Communications >> Print Page (PDF) Formats

Then edit the format marked default and set the margins individually. The margins are set in the Unit of Measure of that page format.