I know that JFreeCharts library is used when generating charts for reports in PDF format.
Is there a way to use any other charting library for PDF format?
First of all, the library used to render chart is not different depending on the export type, ex PDF. Regardless of what document format you target, we use the same charting package to render the chart.
In JR, you can integrate any charting or visualisation package you want, but in order to do that, it means you are going to call that graphic rendering API directly from your report. With our built-in charts, you just configure a chart element and you normally you are not concerned about the JFreeChart API, unless you want some customizations.
If you want to achieve the same simplicity of use, but rely on a different charting library behind the scenes, you would need to implement a JR custom component for it. Just like at Jaspersoft we did with the FusionCharts package, which is available as a JR component in the comercial version of JR Pro.
But if you download the JR project source tree, you can find several samples that show you how to use different charting packages:
I hope this helps.