O.K. now I was also able to reproduce the issue on my box and therefore could find a workaround for solving the problem.
If you go to  on your workspace and do a
mvn clean install -Dtest=foo
The PDF generation fails as described by this ticket description of Hadrian, but if you change to  and do the same, then PDF will get generated successfully (as I also attached once to this ticket while ago). Please note that the PDF generation will be done only if the fastinstall profile is not active so that I did above -Dtest=foo as there'e no test called "foo" I could verify the behaviour much faster.
But what this exactly means? We do exactly the same at two different project directory levels but the behaviour is different! One possible explanation is , that's the DOM implemenataion used by the JDK-XSLT seems to be not thread-safe. When you run maven at the level of  you have so to say a virgin JDK launching up, but running maven under  folder it will take a really long way until the build of camel-manual maven module is reached and exactly at this point the inconsistency (under sun-jdk) by the class
has already happened!
I've already attached the patch
CAMEL-3774.patch to this ticket which resolved the issue on my box even if I run maven at the level of the folder . The solution is pretty simply the HtmlToPdfMojo now supports an optional transformerFactoryClassName field which we set to org.apache.xalan.xsltc.trax.TransformerFactoryImpl not having this thread-safety issue.
Please note that the prerequisite for a proper PDF generation is an available installation of princexml on the PATH. On my iMac I installed it under the PATH
And before calling maven I did
So that princexml can be successfully kicked off by HtmlToPdfMojo (plexus-utils API is used there).
As Christian intends to provide the last 2.7.5 release (see ) I propose to first apply this patch to the 2.7.x branch and if it indeed did the trick we can still apply it afterwards to the 2.8.x and the trunk.
Please let me know what you think about this and if you've got any questions. Thanks!
BTW yesterday I did a full
with all tests passing on the trunk with a proper PDF & HTML inside the generated zip & tarball under the /doc/manual folder
And just noticed that the steps regarding the princexml installation & setup is not mentioned in  at all which we may also want to add as well.