Can I export the performance comparison as a PDF?

Yes, you can export the Performance Comparison data through a report with Optmyzr reporting tools. You can set up the Performance Comparison widget in the Report Designer and export the customized report as a PDF. You can also have this scheduled to hit your inbox at regular intervals.

Which formula does the system use to calculate the change in the metric's performance?

The system calculates the change in metrics performance by using the formula - ((Selection 2 - Selection1)/Selection 1)*100

So, if you are looking at the following comparison for Clicks, the % change is calculated using ((740-580)/580)*100 =28%

Can I compare more than two date ranges in this tool?

If you want to compare more than two date ranges, you can do that using Performance by Time Period widget in Optmyzr Reporting. Read more about it here.