When you create a PDF report of a dashboard, you can use the Optimize PDF for printing or Preserve existing layout in PDF modes.
Optimize PDF for printingedit
Create a print friendly PDF with multiple A4 portrait pages and two visualizations per page.
Preserve existing layout in PDFedit
Create a PDF with the existing layout and size of the Visualization or Dashboard.
When you create a PNG or a PDF report of a visualization, the Optimize for printing option is used.