Not all fonts support Arabic characters in Jasper when run in PDF. We have tested using following fonts successfully with Arabic:
Following is the link to Sample Arabic Report: http://jasper.maxapex.net:8090/JasperReportsIntegration/report?_repName=A13603/TestArabic&_repFormat=pdf&_dataSource=MAX_A13603_392
You may see Arabic characters shown correctly when you preview the report in Jasper Studio which shows the preview in Java mode by default. Make sure to select PDF instead of Java in Jasper Studio Preview window.