Dave Corley has integrated some clever Javascript folding into the PMD XML report! You can see a demo of it here - it's sorted by number of problems per source file, and you can expand/collapse the report for the problems found in the individual files.

The nice part of this is that it's just an XSLT stylesheet linked to the report - so the first two lines of the PMD XML report file look like this:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="pmd_report.xslt"?>

And then all the work is done in the stylesheet. Nice work, Dave!

Note that this is based on Javascript written by Christian Heilmann.