For Opus to search PDF text content, you need an IFilter installed that understands the PDF format. (This will add similar capabilities to Opus, Windows search, and anything else which can use IFilters.)
IFilters typically come with PDF viewers, but only some include them.
PDF-XChange is usually a good option, in our experience.
Adobe Reader also includes an IFilter, but newer versions of it have some problems (not just in Opus), discussed here: Search by content in PDF files
Also note that if File Explorer is able to find things, then you should get the same results by using the search field, at the top-right of the lister (by default), since that uses Windows Search the same as Explorer does. Opus's Tools>Find Files utility does not use Windows Search and may have different results, but will use the IFilter if a working one is installed.