If you want PDF content to be included in the search
mechanism, you need to install the RoboEngine. This component is
part of the RoboHelp Office Professional package with WebHelp Pro.
Any PDF and Word content becomes searchable with this product.
If you are using the plain WebHelp output, the only way to
make that PDF content searchable is to import it into the project
(which converts it to several HTML topic files).