docx4j is intended to run on Java 1.5 (provided JAXB RI is on your path).
To support this, in the 2.7.0 release candidate maven pom.xml, we have:
The objective is to generate a jar which is compatible with Java 1.5, whilst at the same time allowing @Override annotations in the source code on methods which merely implement an interface (Java 6 allows this, but 1.5 gives an error).
When I played around with this, I found that both maven and ant would happily compile to target 1.5 with the pesky @Override annotations present (irrespective of whether source was set to 1.5 or 1.6 iirc).
Eclipse, however, requires that target be not less than source. (Not sure why; I think its semantics are just different) So in Eclipse, source must be 1.6 if you are to have @Override annotations on methods which merely implement an interface.
That is, unless m2eclipse is setting those properties. I found m2eclipse could set source=1.6, target=1.5. I haven't confirmed that exporting a jar in eclipse actually produces a 1.5 jar, but that's a secondary consideration (since we create jars via ant or maven, primarily).