Improve PDFSearchParticipant
It turned out PDFSearchParticipant actually works but it is taken into use for PDF help contributing bundles unless they contain a binding
like this:
<extension
point="org.eclipse.help.base.searchParticipant">
<binding
participantId="org.simantics.help.base.pdf">
</binding>
</extension>
which tells the help system to actually use the specified search participant for the extension-defining bundle.
Improvements to the actual indexing logic are made in this issue.