JasperReports for both PDF and Excel sheets. It does everything one could ever want from a report generator, and then some. Unfortunately, it isn't exactly easy to use. It involves a great deal of XML configuration and manual, absolute positioning.
There used to be iReport for visually composing reports, which to some extent simplified report design. Be aware though, that with one of the more recent versions they've switched to an AGPL. So, you might not be able to use this if you want to keep the source code for yourself.
According to the JasperResports website, Jaspersoft Studio now is their preferred solution for visual report generation.
For just Excel sheets it's probably easier to use Apache POI, which is a robust library for generating and ingesting anything Microsoft Office.
There used to be iReport for visually composing reports, which to some extent simplified report design. Be aware though, that with one of the more recent versions they've switched to an AGPL. So, you might not be able to use this if you want to keep the source code for yourself.
According to the JasperResports website, Jaspersoft Studio now is their preferred solution for visual report generation.
For just Excel sheets it's probably easier to use Apache POI, which is a robust library for generating and ingesting anything Microsoft Office.