There is a configuration element in the PMD plugin that let you set which target JDK to use. Often you want this to be in sync with the configuration for maven-compiler-plugin. The best practice for this is to set up a property and use that in the configuration section for each of the plugins. Here is an example on how to set that up:
<project>
...
<build>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.0.2</version>
<configuration>
<source>${compileSource}</source>
</configuration>
</plugin>
</plugins>
</build>
...
<reporting>
<plugins>
<plugin>
<artifactId>maven-pmd-plugin</artifactId>
<version>3.2</version>
<configuration>
<targetJdk>${compileSource}</targetJdk>
</configuration>
</plugin>
</plugins>
</reporting>
...
<properties>
<compileSource>1.5</compileSource>
</properties>
...
</project>